Basic Scientific Problems of Nickel-Rich Cathode for Lithium-Ion Battery: Destabilization Mechanism and Modification Strategies
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摘要:
层状富镍锂过渡金属氧化物因其高容量、高工作电压等优势是长续航动力电池广泛采用的正极材料。然而,由于不稳定的晶体结构和较差的热力学性,富镍正极材料在反复Li+脱嵌过程中稳定性差,进而导致电池难以长周期服役。本文分析了富镍正极材料表面残锂、阳离子混排、气体释放、不可逆相变、微裂纹等各种导致材料失稳降解的机制,总结了近年来为解决上述问题而采用的元素掺杂、表面涂层、单晶化、浓度梯度结构设计和引入电解质添加剂等改性策略,并展望了未来材料改性策略的方向和应用前景。
Abstract:Nickel-rich layered cathodes with high discharge capacity and high voltage platform are recognized as positive materials used for long-life power batteries. However, due to the fragile lattice and poor thermal property, Ni-rich cathode materials have poor stability during repeated lithiation/delithiation, which makes it difficult for batteries to serve for a long period. In this paper, the mechanism of degradation of stability caused by residual lithium, cationic mixing, gas release, irreversible phase change and micro-cracks for nickel-rich cathode materials is analyzed. The modification strategies adopted in recent years to solve the above problems are summarized, such as element doping, surface coating, single crystallization, concentration gradient structure design and introduction of electrolyte additives. Moreover, the future direction and application prospect of those modification strategy are also discussed.
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锂离子电池(LIBs)自1991年由索尼公司实现商业化以来,受到了学术和工业界的广泛关注[1, 2]。随着能源需求的不断增加,开发兼具高能量密度和安全性的LIBs已迫在眉睫。在技术创新、政府补贴以及电动汽车市场繁荣的刺激下,LIBs已经取得了相当大的进展[3]。LiCoO2[4]、LiFePO4[5]、Li2MnO4[6]等正极材料,因资源稀缺性或能量密度相对较低或稳定性差等因素难以满足长循环动力电池需求[7]。层状富镍锂过渡金属氧化物LiNixM1-xO2(M=Co,Mn,Al等,x≥0.6))具有高能量密度,已成为动力电池正极材料中广泛研究的材料[8]。为进一步降低成本和提高电化学容量,需尽可能降低钴含量并提升镍含量。但是,随着镍含量的增加,材料表界面化学稳定性和结构稳定性逐渐变差(图1(a)),进而导致其安全稳定性和循环性能欠佳 [9]。
研究表明,影响富镍正极性能的因素主要包括以下几个方面:(1)材料制备过程和贮存过程中形成的残锂化合物;(2)材料合成和电化学循环中产生的阳离子混排;(3)电化学循环过程中氧空位诱导的不可逆层状-尖晶石-岩盐相变;(4)表界面重构和电解质副反应导致的O2和CO2气体释放;(5) HF侵蚀导致的活性过渡金属溶解;(6)高SOC和长循环下形成的晶内和晶间裂纹。深入认识和理解这些问题对于开发具有长寿命和高稳定性的LIBs至关重要[10-12]。本文首先探讨富镍正极的失稳机制(图1(b))[13],再对近年来关于缓解性能衰减的措施进行了总结[14-18],并对富镍正极材料的应用前景和发展趋势进行展望。
1. 富镍正极材料的失稳机制分析
1.1 残锂化合物
一般来说,残锂化合物(RLCs)不可避免地存在于富镍正极材料中,且其含量随着镍含量的增加而增加[19]。RLCs的产生可能有两个来源。一是合成过程为避免高温下锂蒸发加剧的阳离子混排而添加的过量LiOH。多余LiOH会以Li2O的形式残留在材料表面,导致材料RLCs含量增加[20]。二是富镍正极材料暴露在空气中时,颗粒表面的Li2O会与空气中的H2O和CO2反应逐渐转化为LiOH和Li2CO3,LiOH进一步与CO2反应生成LiHCO3和Li2CO3 (图2(a)),相关反应方程式如下[21]:
Figure 2. (a) The change of the surface for Ni rich cathode after exposure in air [21]. (b) The change of the microstructure and composition of the solid-electrolyte interface for Ni-rich cathode during cycle [22]. (c) The change of surface of Ni-rich cathode after washing, drying and subsequent treatment [23].Li2O+H2O/CO2→2LiOH/Li2CO3 (1) LiOH+CO2→LiHCO3 (2) 2LiOH+CO2→Li2CO3+H2O (3) 因此,RLCs通常以Li2O、LiOH和Li2CO3的混合物形式被检测出来,它们的比例随着储存条件(如湿度和时间)的变化而改变。
RLCs容易吸收水分,导致正极浆料凝胶化,给电极的制造过程带来困难。LIBs常用电解液组分为含1 M LiPF6的碳酸二乙酯(EMC)、碳酸二甲酯(DMC)、碳酸乙烯酯(EC)(体积比为1∶1∶1)的混合有机溶剂。在电化学循环过程中,正极材料和电解液可能发生的副反应如下所示[22]:
2LiOH+LiPF6→3LiF+POF3+H2O (4) LiPF6→LiF+PF5 (5) PF5+H2O→POF3+2HF (6) Li2CO3+2HF→2LiF+H2O+CO2 (7) 2CO2+2Li++e−→2Li2CO3+CO (8) LiOH、Li2CO3在DMC和LiPF6的作用下分解产生CO2、LiF与LixPOFy (图2(b))。其中LiF的离子传输性能不佳,降低富镍正极颗粒表面的离子传输动力学;而副反应产生的CO2、CO气体可能导致电池膨胀影响LIBs的安全性能[23]。因此,额外的工艺处理以降低富镍正极的残锂化合物含量十分必要。目前,洗涤、表面包覆等方法常被用来去除残锂化合物,以改善正极材料的电化学性能,其中最高效的方法是通过水洗(图2(c))[24]。
然而,水分子容易与内部的活性组分发生反应并破坏富镍正极的层状有序结构,水洗会导致结构破坏和容量损失[25]。此外,水洗过程还增加富镍正极的空气敏感性。因此,需要仔细控制洗涤参数和工艺流程。Xiong等人[26]研究LiNi0.8Co0.1Mn0.1O2水洗处理前后的性能差异发现:与未经处理的样品相比,经过洗涤处理后的样品存储容量有所降低,但表现出更好的容量保持率、增强的电极稳定性和减少的气体逸出。Abraham等人[27]和Liu等人[28]指出,在洗涤过程中由于LiNi0.8Co0.1Mn0.1O2与H2O反应,在表面区域附近会产生类似NiO的薄层。当热处理温度较低时,NiO层不能被重新氧化,表现出降低的储锂容量。在O2气氛中经过700℃的热处理后,表面NiO层与未去除的残留物Li2O/LiOH发生反应并被重新氧化,可改善材料的循环性能。
1.2 阳离子混排
LiNixM1-xO2(M= Co,Mn,Al等,x≥0.6)正极材料具有典型的六方α-NaFeO2层状结构,属于R-3 m空间群,由锂离子和TM离子的交替层组成,被氧原子层隔开。Li+/Ni2+混排(即镍占据锂的3 b位点,而锂占据镍TM层中的3 a位点)不仅在材料合成过程中发生,而且在电化学循环过程中也会发生[29]。当Ni含量增加到较高值时,八面体位中Li+/Ni2+混排导致的结构无序成为影响性能关键问题之一。Li+/Ni2+无序化不仅是富镍正极材料固有的结构属性,还影响倍率性能、放电容量、循环稳定性和热稳定性。
Li+/Ni2+混排会阻碍锂离子扩散动力学,从而影响其倍率性能和放电容量。通常认为Li+在层状富镍正极的晶体结构扩散有两种路径。一种是Li+通过直接跳过氧哑铃从一个八面体位点扩散到下一个八面体位点,这被称为氧哑铃跳跃(ODH)。另一种是Li+通过Li扩散留下的双空位跳跃到过渡金属围绕的中间四面体位点,这被称为四面体位点跳跃(TSH) (图3(a))。两种扩散路径的扩散能垒不仅取决于激活状态下Li+和TM离子之间的静电作用,而且还取决于应变效应(ODH的氧哑铃鞍点的大小和TSH的四面体位点的大小),导致ODH路径和TSH路径的扩散能垒分别为0.75~0.91 eV和0.36~0.54 eV[30]。
图 3 (a)具有一对Li+/Ni2+混排的层状富镍正极材料的α-NaFeO2型结构示意图(左图)和Li+扩散途径的氧哑铃跳跃(ODH)和四面体位点跳跃(TSH)示意图(右图) [30]; (b)NMC811中 I003:I104值与阳离子混排程度的关系[33]。Figure 3. (a) Schematic diagram of the layered Ni rich cathode with α-NaFeO2 type structure and one pair of Ni/Li exchange (left panel) and the ODH (oxygen dumbbell hopping) and TSH (tetrahedral site hopping) types for Li ion diffusion pathways (right panel) [30]. (b) Relationship between peak intensity ratio of (003) to (104) and percentage of the cation mixing in the case of NMC811 [33].反位Ni2+阻碍锂离子扩散动力学的原因有以下几种:(1)富镍正极材料中的锂离子通过锂层进行二维扩散,反位Ni2+会占据锂离子扩散必经通道。(2)反位Ni2+的高电荷状态可能导致迁移的Li+有更强的库伦排斥力,导致较低的Li迁移率[30]。(3)由于ODH和TSH扩散的激活能与应变效应密切相关,而应变效应由c轴参数(Li层空间)决定,强的O-Ni2+-O的相互作用挤压Li层空间,导致锂扩散的扩散能垒更高。(4)在首次充电过程中,占据Li位点的Ni2+离子被氧化成尺寸更小的Ni3+离子,这将导致这些镍离子周围局部收缩,锂离子很难进入周围的位置[31, 32]。
在TM层中,当价态为+2、+3和+4的Ni离子被Li+取代时,相关TM离子的局部库仑相互作用发生改变,并且TM离子之间的超交换相互作用网络也由于反位Li+上缺少局部d轨道电子而被破坏[33-35]。因此,库仑作用和超交换相互作用都会引起晶格畸变,导致相结构中的各向异性应力。Li+/Ni2+高度无序化时,晶胞的各向异性应力随着Li+/Ni2+无序化的增加而急剧增加。脱锂过程中,Li+/Ni2+混排行为将导致不可逆的相结构转变 [36]。在晶粒无择优生长的情况下,XRD的(003)和(104)峰强度比可反应阳离子混合程度。随着阳离子混排量的增加,I003/I104的比值下降(图3(b))[33]。
对于Li+/Ni2+无序混排带来的不利影响,通常可采用阳离子取代(掺杂)的方法来控制。以Li过量的层状氧化物为例,当部分TM阳离子被Li+取代时,需要氧化TM离子以保持电荷中性[37, 38]。被氧化的金属元素通常是镍元素,因为Ni2+的氧化在能量上比Mn4+和Co3+更有利。除此之外,还可以通过调整合成过程中的热力学和动力学条件以及表面反应来控制阳离子混排程度。由于Li+/Ni2+无序化通常发生在材料制备过程中,因此在合成过程中调节热力学和动力学条件(煅烧温度和保温时间)可控制Li+/Ni2+混排程度[39, 40]。从热力学和动力学的角度来看,一旦温度高到足以驱动有序化,较低的加热温度和较长的保温时间有利于产品的Li+/Ni2+有序化。表面反应对富镍正极整体阳离子有序/无序动力学有显著影响[41]。在合成过程中,若空气中的CO2和原料(LiOH)发生反应在颗粒表面形成Li2CO3,则需要足够的烧结温度来达成反位Ni2+氧化为Ni3+以及Li2CO3的分解,从而促进反位Ni3+迁移回TM层和更多的Li+从表面扩散到本体占据3 b位置。同时,过低烧结温度会导致更多的Li+/Ni2+混排(Ni2+难以氧化)和更多的RLCs形成;但当烧结温度过高时,Li/O的流失速度会大大加快,从而促进Li+/Ni2+混排[42]。
1.3 气体释放
产气是影响LIBs安全性最显著的问题。产气过程涉及的反应通常为放热反应,从而引发一系列连锁效应,最终导致电池爆炸或自燃。产气过程生成的气体一般为CO2和O2。气体的起源来自三个方面:(1)表面未反应的RLCs的分解以及与电解质的反应;(2)富镍正极由阳离子混排等结构缺陷和后续电化学过程中的相转变过程引起的晶格氧损失;(3)电解质直接氧化[43]。由于表面缺陷的存在,高荷电状态会加速气体的产生。
普遍认为,富镍正极和电解液之间的复杂相互作用会形成气态分解产物。除来源于表面杂质外,气体生成也可能来自富镍正极表面的相变。Streich 等人[44]发现部分氧化的表面氧(如超氧或过氧化物)对有机碳酸溶剂具有强烈生成CO2的反应性(图4(a))。同时,结构变换引起的晶格氧损失会导致O2生成,并增加电池阻抗,,降低电池循环寿命。图4(b)显示了不同镍含量对气体释放的影响,较高的镍含量有利于气体形成。图4c是NCM111和NCM811在三种不同状态下的电子态密度(DOS)图,其中“M-O2*”是表面物质的能量状态,CO2和O2气体的形成速率由Ni-O2*表面状态的电子消耗速率决定。这些发现进一步明确了富镍正极材料表界面的物质组成与结构/表面稳定性之间的关系。
图 4 (a)富镍正极表面重建以及CO2和O2形成的选定反应的示意图。(b)首次充电期间,CO2和O2的释放和Ni氧化状态的关系。(c)NCM111和NCM811在完全锂化、最大镍氧化和完全脱锂时Mn-O2*、Ni-O2*和Co-O2*的表面状态[44]。Figure 4. (a) Schematic overview of selected reactions resulting in surface reconstruction as well as CO2 and O2 formation. (b) Dependence of CO2 and O2 evolution on the state of Ni oxidation during 1st charge. (c) NCM111 and NCM811 extended by Mn−O2*, Ni−O2*, and Co−O2* surface states at full lithiation, maximum nickel oxidation, and complete delithiation [44].1.4 不可逆相转变
富镍正极相变会导致材料表界面重构,并从表面逐渐扩展至体相。表界面重构形成的表面层会阻碍锂离子和电子的传输,恶化循环性能,导致整体容量下降。如图5(a)所示[45],相转变过程中,原始晶体结构从层状结构变为缺陷尖晶石状结构,最后变为无序岩盐相结构[46, 47]。典型的容量微分曲线(图5(b))表明H2→H3的相转换是该过程的原因[48, 49]。
其中,形成的NiO岩盐相具有较差的Li+传输动力学,这是由于NiO岩盐相既不具有电化学活性,也不具有离子导电性,其存在会导致形成厚而高电阻的表面层,从而增加电池的阻抗,导致锂离子扩散受阻,电导率降低。为了解富镍正极材料的容量衰减机制,Manthiram等人[50]对循环后的正极颗粒表面进行了透射电子显微研究。在4.3 V的电压下循环100周后,在NCM622的表面观察到薄的类NiO结构,受损的表面层被限制在3 nm的厚度(图5(c))。相比之下,NCM90正极表面衰退明显更广泛,类NiO层的厚度增加到约5 nm。由于NCM90正极表面存在大量热力学不稳定的Ni4+,其不仅使电解质容易氧化,且易被还原为Ni2+,使晶格氧被氧化形成O2,以保持电荷中性。大量氧的逸出会产生大量氧空位,降低过渡金属阳离子迁移的活化能垒,从而加剧岩盐结构的形成。此外,Grey等人[51]提出:当电极充电至高于约75%阈值的荷电状态时,富镍正极的层状结构和岩盐的表面重建层之间的晶格失配是疲劳退化的主要原因。
以下等式描述了晶体结构的转换过程[19]:
3NiO2(layered)→Ni3O4(spinel)+2[O] (9) Ni3O4(spinel)→NiO(rock - salt)+2[O] (10) 1.5 过渡金属溶解
过渡金属离子从正极溶解到电解质中是一个不可避免的过程,这会间接导致主体结构中锂离子插入位点的损失,导致容量下降。一般来说,正极过渡金属的溶解与有机电解质中的HF和HClO4形成密切相关。普遍认为,电解质溶剂在制造过程中难以避免痕量水的存在,这些水分子与锂盐(LiPF6或LiClO4)发生反应会产生酸性物质。存在于电解液中的H+攻击正极,诱发过渡金属的溶解。相关反应可描述如下[21]:
LiPF6→LiF↓+PF5 (11) PF5+H2O→POF3+2HF (12) LiClO4+H2O→LiCl+HClO4 (13) 其中HF和HClO4的产生还可能来自残余锂化合物与有机溶剂中PF−6或ClO−4之间的反应。循环过程中晶格氧损失可导致低价离子的形成,也会促进过渡金属溶解,这主要是低价态的金属离子比高价态的金属离子更易溶解在电解液中。此外,过渡金属离子也可迁移到石墨负极上被电化学还原成金属打破原有的正极/阳极容量比,或在石墨表面参与形成SEI,致使阻抗增加,这也是导致电池容量和工作电压下降的原因之一[19]。
1.6 晶内和晶间裂纹
微裂纹引起富镍正极颗粒的机械失效被广泛认为是影响电池性能衰退的重要因素。根据微裂纹存在的位置,裂纹可分为晶内裂纹或晶间裂纹(图6(a)和6(b) )[52]。
图 6 (a)100次循环的NMC811晶粒上的晶内裂纹。RS:岩盐。SRL:结构重建层;(b)原始的和循环的NMC811的低倍STEM-HAADF图像,蓝色箭头标记的晶间裂缝[52];(c, d) 晶内裂纹(c)和晶间裂纹(d)的形成和发展机制[53]。Figure 6. (a) Intragranular crack observed on the 100 cycled NMC811 particle. (b) Low magnification cross-sectional STEM-HAADF images of pristine and cycled NMC811, marking with the intergranular cracks by blue arrows [52]. (c, d) The formation and development mechanism of intragranular crack (c) and intergranular crack (d) of Ni-rich cathode [53].晶内裂纹通常由Li+/Ni2+反位缺陷和晶格紊乱以及离子之间的库伦斥力引起。图6c显示了晶内裂纹的形成发展过程。原始颗粒中的晶格缺陷在循环过程中不断增长,最终发展成的纳米级裂纹。晶内裂纹的形成与H3相的往复转化、不均匀SOC结构有关。伴随着充放电过程H3相的往复转化,一次晶粒发生膨胀/收缩。同时,由于材料中不均匀的应力变化(面外应力小于面内应力)材料反复锂化/脱锂过程中不平衡的应力积累最终导致微裂纹的产生[53]。再者,Li+/Ni2+混排会形成脆弱的岩盐相区,该相区也会受到拉伸应力,诱导产生晶内裂纹。此外,不均匀脱锂产生的异质SOC结构也会带来局部层状剥落和裂纹成核[54]。
晶间裂纹的形成主要是由一次晶粒各向异性的体积变化导致的,并从颗粒中心向表面扩散(图6(c))。晶间裂纹的形成与晶内裂纹相似,相当于是多个一次晶粒之间的应力累积。脱(嵌)锂过程中,H2↔H3相变使一次晶粒晶格参数各向异性突然收缩和膨胀,并且随着Ni含量增加而加剧[55]。同时,非放射状二次颗粒的一次晶粒通常是不规则的多面体,导致颗粒内因相变而产生的应力方向随机分布,局部区域会呈现应力集中。因此,在充放电过程中,富镍正极二次颗粒内应力集中的晶界处形成晶间裂纹,从而破坏二次颗粒结构的机械完整性。
相较于晶内裂纹,晶间裂纹的产生和扩展被认为是富镍正极性能衰退更重要的因素。一方面,晶间裂纹会减少一次晶粒之间的连接,降低二次颗粒内部一次晶粒之间的离子/电子传导,带来更不均匀的荷电状态。不均匀的荷电状态会降低材料的活性,导致颗粒粉化和碎裂。另一方面,微裂纹的产生暴露二次颗粒内部更多的活性位点为相变、腐蚀和电解液分解副反应创造新的反应位置,从而加速电池的容量衰减[56]。随着Ni含量、循环次数和SOC的增加,微裂纹开裂程度将更加严重。Sun研究了不同Ni含量富镍正极的微裂纹演变[57],发现镍含量高于80%的富镍正极在脱锂过程中具有更明显的各向异性收缩。如何在保持高Ni含量的同时,采取有效措施缓冲一次晶粒/二次颗粒之间的体积应变,减少微裂纹的产生,保持电极材料的整体性是提升富镍正极电化学性能的关键。
2. 富镍正极材料的稳定性提升策略
针对材料失稳机制,富镍正极稳定性提升策略主要包括元素掺杂、表面包覆、单晶化、浓度梯度结构设计和引入电解质添加剂等。
2.1 表面包覆
表面包覆被认为是提升富镍正极稳定性简单而有效的方法。通过在材料表面涂覆一层保护层抑制或减弱正极电极与电解质之间的副反应,从而减少过渡金属的溶解,增强表界面稳定性,提高电化学性能。各种涂层物质,如金属氧化物(Al2O3[58]、CeO2[59]、ZrO2[60]、TiO2[61])、氟化物( LiF[62],AlF3[63]))和磷酸盐(AlPO4[64],FePO4[65] )、导电聚合物( PMMA[66],PAN[67]))和快离子导体( Li3PO4[68],Li4Ti5O12[69])等被使用作为富镍正极包覆层。金属氧化物、氟化物及磷酸盐涂层能够隔离正极和电解质之间的接触,有效地防止HF腐蚀,提高材料的电化学特性。然而,这些物质的离子导电性和电子导电性较差,会增加界面阻抗,降低电化学动力学。因此,具有优异离子或电子导电性的导电聚合物和具有良好离子导电性的快离子导体逐渐应用于包覆改性富镍正极,并表现出优异的电化学性能。Zhang等人[68]沿着二次颗粒内一次晶粒的晶界注入Li3PO4 固态电解质,显著提升了结构和界面稳定性(图7(a)),提升电池长循环稳定性。对于包覆策略而言,选择合适的涂层材料和包覆厚度将有助于提高材料电化学性能。Zheng等人[66]采用PMMA+PVDF复合纳米层对NCM811材料进行表面改性(图7(b)),发现该特殊的界面结构不仅有效地将镍离子锚定在材料表面,而且极大地抑制了其向电解质的溶解。Xu等人[70]利用氧化化学气相沉积(oCVD)技术将导电聚合物3,4-乙烯二氧噻吩(PEDOT)注入晶界和表面,发现经过长时间循环后,利用PEDOT修饰晶界能显著减少微裂纹,抑制结构从层状向尖晶石/岩盐结构的演变(图7(c))。
2.2 元素掺杂
微裂纹的产生和扩展归因于H2↔H3相变过程中晶格反复膨胀/收缩引起的各向异性体积变化。元素掺杂是调节富镍正极结构并提升其电化学性能的有效方法,并被广泛用于控制一次晶粒的尺寸、表面微结构和晶粒晶体结构,以优化电化学性能。根据掺杂元素类型可分为阳离子掺杂、阴离子掺杂等。不同的掺杂类型(元素类型、掺杂浓度和取代位点等)具有不同的改性效果。常用的阳离子掺杂元素有Ta、Na、Mg、Ca、B、Al、Ti、Zr等[71-76]。其中,Al3+、B3+、Zr4+、Ti4+、Nb5+、W6+等元素已被大量结果证明可有效提高富镍正极材料的结构稳定性。
高价元素掺杂能够有效调整一次晶粒的颗粒形态从而实现堆积方式的调控。例如,掺杂Sb5+后,NC90和NCA89随机排布的多边形一次晶粒变成了具有较高长径比的晶粒。这主要是高温锂化过程高价元素在晶界处发生偏析,抑制了氢氧化物前驱体球形二次颗粒中一次晶粒的粗化 (图8(a))[72]。Kim等人[71]合成了掺杂1 mol%Ta的Li[Ni0.90Co0.09Ta0.01]O2,该材料具有(003)晶面径向排列的一次晶粒(图8(b)),从而有效地耗散深度充电状态过程出现的内部应变。同时,高价元素取代Ni3+会诱导Ni2+离子在Li层中有序占据并稳定脱锂材料的晶体结构。该材料在2.7-4.3 V下循环100周后相较于其他元素掺杂容量保持率提升显著,在完全放电深度下循环2000次后,容量保持率为90%,正极能量密度>850 Wh kg-1。元素掺杂能有效提高富镍正极的电化学性能,但掺杂量需要精细控制。同时,选择经济且高效的元素进行掺杂是必要的。单元素的掺杂效果有限,但实现多元素的均匀掺杂具有挑战性[77]。根据制备工艺方法的不同,掺杂元素引入可在共沉淀阶段、锂混合阶段和正极包覆二烧后取代阶段。最后一种方法难以实现均匀掺杂,但共沉淀法由于掺杂剂的热力学扩散和偏析性质不同,仍仅适用于低价元素。
2.3 浓度梯度结构设计
当镍含量高于80%时,富镍正极和电解质之间的界面会变得不稳定,容易发生结构转变,最终导致电池性能衰减。构建二次颗粒由内至外Ni含量逐渐降低的浓度梯度结构是解决高镍含量导致结构衰退的方法之一。浓度梯度结构可以同时利用富镍核的高容量和中低镍壳的稳定性提高富镍正极的结构稳定性[78, 79]。Sun等人[80-84]首先提出富镍核以保证正极的容量,并以较低镍作为表面层提升表界面的稳定性。根据Ni含量的分布形式(图9(a)-(d)),可分为四种梯度类型:(1)在核壳界面具有浓度梯度的核-壳结构(CS) [80];(2)在壳层具有Ni浓度梯度的核-梯度壳结构(CSG) [81, 82];(3)从中心到表面的全浓度梯度核-壳结构(FCG)[83];(4)双浓度梯度的双斜率浓度梯度结构(TSFCG)[84]。Sun等人以NCM811作为核, LiNi0.5Mn0.5O2作为壳构筑了CS型富镍正极,该材料全电循环500周后容量保持率为98%。性能提升的原因在于表面的稳定的低镍组分LiNi0.5Mn0.5O2[80]。然而,界面处Ni浓度的急剧变化导致核壳界面晶格失配,Li+的反复脱嵌导致内核和外壳中的晶格收缩/膨胀不同步,最终导致相分离。CSG结构通过连续共沉淀过程实现具有Ni含量渐变的壳层,可以有效缓解CS核壳正极界面局部应力的累积。进一步将浓度梯度扩展至二次颗粒中心能得到FCG结构,Ni浓度在径向上由内至外呈线性下降,同时Mn/Co元素在径向上增加。尽管其比容量略低于核组分的富镍正极,FCG结构由于避免了CS或CSG结构存在的晶格不匹配问题从而大幅度提升循环稳定性。前驱体形态、烧结步骤是调控径向浓度梯度获得FCG正极的关键。TSFCG结构在中心区域平缓梯度,而靠近表面区域陡峭梯度。中心区域的平缓浓度梯度是可使内部Ni含量最大化,外部Mn/Co元素的陡峭梯度可提升近表面区域的稳定性。
Sun等人[85]发现CSG-NCMA90二次颗粒的独特微观结构显著提高其机械稳定性,并抑制循环过程中微裂纹形成。当二次颗粒在高充电状态下经受高应力时,径向排列的一次晶粒允许CSG-NCMA90正极均匀收缩(图10(a)),以有效消散内部应变并抑制微裂纹的形成。相比之下,微裂纹容易在常规NCMA90正极随机取向的一次晶粒之间成核,并沿着大的等轴一次晶粒间的晶界快速扩展到二次颗粒的外表面。此外,CSG-NCMA90正极的薄且细长的一次晶粒耐受电解质侵蚀,抑制了表面衰退。在100% DoD和60% DoD条件下
1000 次循环后容量保持率分别为90.7%和94.1%(图10(b))。但梯度结构材料的工业应用面临一些问题。首先,大规模合成浓度梯度前驱体难控制。在锂脱出/嵌入时,二次颗粒内变化的体积收缩/膨胀仍会加速颗粒粉碎,对长期循环性能产生不利影响[86]。为了解决这个问题,Kim等人[87]将Al引入具有平均组成为LiNi0.61Co0.12Mn0.27O2 (FCG61)的浓度梯度正极,其中Ni和Mn的浓度被设计成在正极颗粒内连续变化。铝取代使Al-FCG61正极在3000 次循环后(2.7-4.2 V,30℃,1 C)也能保持其初始容量的84%(图10(c))。Al取代强化了晶界,从而延迟晶界处微裂纹的成核,减少了阳离子混合,并抑制了可能引发微裂纹的有害相的形成。此外,一些高价元素梯度掺杂也被引入到正极材料的合成中以缓解富镍正极材料在长循环过程中的衰减[88-91]。Wang等人[91]通过在NC90正极材料中梯度掺杂Te6+,成功制备了掺杂1.0 mol% Te的NC90,显著提升了其结构和电化学性能。在0.5 C电流下循环100次后,容量保持率超过95%。Te的引入不仅细化了颗粒,增加了(003)晶面间距,提高了Li+扩散速率,还降低了Li+/Ni2+混排。同时,Te-O键的强化作用稳定了晶格,抑制了氧释放和H2→H3相变,增强了材料的热稳定性和循环性能。图 10 (a) NCM90、NCMA90、CSG-NCM90和CSG-NCMA90正极的横截面SEM图像; (b) NCMA90和CSG-NCMA90正极在100% DoD和60% DoD下循环的长期循环性能[85]; (b)掺铝的浓度梯度LiNi0.61Co0.12Mn0.27O2长循环性能[87]。Figure 10. (a) Cross-sectional SEM images of the as-prepared NCM90, NCMA90, CSG-NCM90, and CSG-NCMA90 cathodes. (b) Long-term cycling performance of NCMA90 and CSG-NCMA90 cathodes cycled at 100% DoD and upper 60% DoD using pouch-type full cells [85]. (c) Long-cycle-life of Al-doped LiNi0.61Co0.12Mn0.27O2 (≥3000 cycles with 100% depth of discharge) [87].2.4 单晶化
传统富镍正极为一次晶粒聚集而成的多晶球形二次颗粒。由于锂离子脱嵌导致晶格参数变化各向异性,一次晶粒随机排列会导致强烈的晶界应力,引发晶间裂纹形成。晶间裂纹形成不仅导致一次晶粒之间电子转移失败,还加速电解质侵蚀。研究表明,将材料单晶化是降低晶界应力的有效策略[92, 93],提升机械强度和更均匀的电化学反应。Yang[94]等人对粒径为3-6 µm的单晶LiNi0.83Co0.11Mn0.06O2(SC-NCM)进行了全面研究。发现SC-NCM具有较好的循环性能,长期循环后甚至没有裂纹(图11(a)-(b))。单晶颗粒显著防止了各向异性应力引起的定向体积变化进而导致的晶间裂,从而减轻了电极/电解质间有害作用。因此,增强的界面稳定性有效地防止了层状相向岩盐相的不可逆相变,显著提高了结构稳定性和循环性能。然而,单晶合成的条件较为苛刻,在低温条件下几乎无法得到大的单晶颗粒,而在高温下只有配锂量较高时才能获得大的单晶颗粒。同时,过高的烧结温度会加剧Li/Ni混排并影响晶体结晶性,造成循环性能下降。此外,高温煅烧会造成Li挥发,需补充适量的锂源。
图 11 (a)富镍多晶和单晶正极在延长循环过程中的裂纹演化和内部形态差异示意图。(b) 55℃时富镍多晶和单晶在2.75-4.4 V电压范围内的循环性能[94]。Figure 11. (a) Schematic illustration of crack evolution and the internal morphological difference for polycrystal and single crystal. (b) Cycling performance polycrystal and single crystal Ni-rich cathode at 55℃ in the potential of 2.75-4.4 V [94].2.5 电解质添加剂
电极表面的固体/电解质界面膜(正极侧称为CEI膜,负极侧称为SEI膜),主要成分为电解质组分分解反应产生的各种有机/无机化合物。考虑到电解液是CEI膜的主要来源,通过引入电解质添加剂(质量比或体积比通常小于5%)构筑稳定的富镍正极CEI膜是有效的策略[95, 96]。电解质添加剂能够有效去除有害的组分(HF或H2O等)或原位构建稳定的CEI膜,以抵抗电解液的侵蚀。均匀、薄而致密的CEI膜能促进Li+扩散,降低界面阻抗,缓解过渡金属溶解,覆盖催化电解质分解活性位点。电解质添加剂通常比电解液组分具有更高的HUMO(最高已占据分子轨道)能量,比电解液组分优先分解调控CEI膜组分。按照官能团进行分类,包括砜类、磷酸盐和亚磷酸盐、含硅官能团、硼酸盐结构、氟化、VC和酯结构等添加剂。
Sun等人[97]提出了一种能有效稳定锂金属负极和富镍NCM正极界面的双功能添加剂己二腈(C6H8N2)。添加1 wt%的己二腈可形成稳定且导电的金属负极/电解质界面。同时,由于Ni4+与己二腈中腈基之间的强配位,电解质与富镍正极表面之间的寄生反应减少。添加C6H8N2的Li[Ni0.73Co0.10Mn0.15Al0.02]O2电池具有优异的电化学性能,在高容量负荷1.8 mAh cm−2 830次循环中实现了75%容量保持率(图12(a))。Choi等人报道了使用3-(三甲基硅基)-2-恶唑烷酮(TMS-ON)作为电解质添加剂的工作原理[98]。含有TMS-ON的电池循环400周后的容量保持率为80.4%,循环后的容量为154.7 mAh g-1。
正常情况下,LiPF6的存在形式包括:阴离子配对的LiPF6、分解的LiF和不稳定的PF5。PF5作为一种强路易斯酸,PF5会诱导EC的开环反应和在正极/负极侧表面形成CEI/SEI膜。此外,高活性的PF5容易与H2O反应生成高活性的POF3、HF和磷酸化合物(HPO2F2、H2PO3F和H3PO4)。电解液中的HF不仅会造成过渡金属溶解,还会对正极的CEI膜造成伤害(图12(b))。TMS-ON中的N-Si键,倾向于与HF反应,并且高极性环状结构能够有效与Li+结合,从而提高LiPF6的解离度和减少离子对LiPF6的含量。此外,TMS-ON中的Si原子能够与HF结合最终生成三甲基氟化硅的中间体,从而有效清除电池体系中有害的HF和H2O组分(图12(c))。目前,对与电解质添加剂的研究仍处于实验阶段,有必要将基础研究与应用研究结合,选择高效的功能添加剂。成膜添加剂用量低,容易过早消耗。单一添加剂往往不能兼顾电解质溶液的多功能指标,开发多元添加剂将成为未来研究的重点。此外,研究人员需结合和开发多种表征技术深入了解添加剂的化学结构对CEI结构和电池性能的影响。筛选和定制特殊的添加剂构建稳定CEI膜,进而提升LIBs的循环性能和安全性十分重要。
3. 结 论
随着对高能量密度、长循环寿命和高安全LIBs需求的增长,正极材料的高电压和长循环性能要求逐渐增加。富镍正极具有高容量、良好的倍率性能和合理的成本,成为高能量密度动力电池主要的候选材料。本文综述了富镍正极存在的主要电化学降解机制以及改性方法的最新进展。富镍正极面临的挑战主要包括正极表面残留的锂化合物、阳离子混排、表面结构重建、气体释放、过渡金属溶解、微裂纹等。应对材料降解机制的策略包括元素掺杂、表面涂层、单晶化、浓度梯度设计和电解质添加剂等。总体而言,研究人员已经提出了许多改性策略来改善长循环过程富镍正极材料的性能,但实现长循环、高容量富镍正极材料的应用仍然需要克服许多挑战。通过先进的分析技术深入了解材料,有助于设计合理的改性策略,提高材料的容量和循环稳定性。此外,掺杂和包覆策略的内在机制,单晶或浓度梯度的富镍正极材料的合成,以及多功能电解质添加剂的使用仍需要深入研究,以采用多种改性措施相结合的方式缓解富镍正极材料电化学衰减,满足车网互动新形势下对富镍正极材料长循环性能的需求。
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图 2 (a) 富镍正极在空气中暴露后的表面变化示意图[21]; (b)富镍正极循环过程表面SEI膜微结构和组分变化[22];(c)富镍正极在水洗、干燥和后续热处理过程中的表界面变化示意图[23]。
Figure 2. (a) The change of the surface for Ni rich cathode after exposure in air [21]. (b) The change of the microstructure and composition of the solid-electrolyte interface for Ni-rich cathode during cycle [22]. (c) The change of surface of Ni-rich cathode after washing, drying and subsequent treatment [23].
图 3 (a)具有一对Li+/Ni2+混排的层状富镍正极材料的α-NaFeO2型结构示意图(左图)和Li+扩散途径的氧哑铃跳跃(ODH)和四面体位点跳跃(TSH)示意图(右图) [30]; (b)NMC811中 I003:I104值与阳离子混排程度的关系[33]。
Figure 3. (a) Schematic diagram of the layered Ni rich cathode with α-NaFeO2 type structure and one pair of Ni/Li exchange (left panel) and the ODH (oxygen dumbbell hopping) and TSH (tetrahedral site hopping) types for Li ion diffusion pathways (right panel) [30]. (b) Relationship between peak intensity ratio of (003) to (104) and percentage of the cation mixing in the case of NMC811 [33].
图 4 (a)富镍正极表面重建以及CO2和O2形成的选定反应的示意图。(b)首次充电期间,CO2和O2的释放和Ni氧化状态的关系。(c)NCM111和NCM811在完全锂化、最大镍氧化和完全脱锂时Mn-O2*、Ni-O2*和Co-O2*的表面状态[44]。
Figure 4. (a) Schematic overview of selected reactions resulting in surface reconstruction as well as CO2 and O2 formation. (b) Dependence of CO2 and O2 evolution on the state of Ni oxidation during 1st charge. (c) NCM111 and NCM811 extended by Mn−O2*, Ni−O2*, and Co−O2* surface states at full lithiation, maximum nickel oxidation, and complete delithiation [44].
图 5 (a)富镍正极循环过程中从表面到内部的结构演变示意图[45]; (b)富镍正极材料典型dQ/dV曲线[48];(c) 4.3 V100次循环后的 NCM 622和 NCM 90的TEM表面图像和傅立叶变换 [50]。
Figure 5. (a) Schematic illustrations of the structural evolution of Ni-rich cathode from the surface to bulk during the cycling [45]. (b) Typical dQ/dV curves of Ni-rich cathodes [48]. (c) TEM images and corresponding Fourier Transform of NCM 622 and NCM 90 charged to 4.3 V after 100 cycles [50].
图 6 (a)100次循环的NMC811晶粒上的晶内裂纹。RS:岩盐。SRL:结构重建层;(b)原始的和循环的NMC811的低倍STEM-HAADF图像,蓝色箭头标记的晶间裂缝[52];(c, d) 晶内裂纹(c)和晶间裂纹(d)的形成和发展机制[53]。
Figure 6. (a) Intragranular crack observed on the 100 cycled NMC811 particle. (b) Low magnification cross-sectional STEM-HAADF images of pristine and cycled NMC811, marking with the intergranular cracks by blue arrows [52]. (c, d) The formation and development mechanism of intragranular crack (c) and intergranular crack (d) of Ni-rich cathode [53].
图 7 (a)通过ALD涂覆和退火在LiNi0.76Mn0.14Co0.10O2二次颗粒内注入的LPO涂层的示意图[68];(b) SC-NCM涂覆PMMA表层的工作机制示意图[66];(c) PEDOT涂层提升NCM界面稳定性的作用机制[70]
Figure 7. (a) Schematic diagram of the evolution of LPO coating injected on LiNi0.76Mn0.14Co0.10O2 secondary particles after ALD coating and annealing [68]. (b) The working mechanism of PMMA modified SC-NCM[66]. (c) The mechanism of PEDOT coating for enhancing the interfacial stability of NCM[70].
图 8 (a)具有不同晶粒长宽比的NC90、NCA89和NCSb89的裂纹衍生情况[72]; (b) Al, B, Ta/W掺杂的形貌演变结果[71]。
Figure 8. (a) Cracking of LiNi0.90Co0.10O2 (NC90), LiNi0.885Co0.10Al0.015O2 (NCA89), and LiNi0.89Co0.10Sb0.01O2 (NCSb89) particles with difference primary grains after the cycle [72]. (b) Morphologies evolution of Al, B, Ta/W doped NC90[71].
图 9 不同的梯度结构模型: 核壳结构(CS) [80] (a),核-梯度壳结构(CSG) [81] (b),全浓度梯度(FCG) [83](c),双浓度梯度(TSFCG) [84] (d)和对应正极形貌、元素分布及电化学性能。
Figure 9. Different Ni rich cathode with: core–shell (CS) [80] (a), core–shell gradient (CSG) [81] (b), full concentration gradient (FCG) [83] (c), two-slope full concentration gradient (TSFCG) [84] (d) and their morphology, composition and electrochemical performance.
图 10 (a) NCM90、NCMA90、CSG-NCM90和CSG-NCMA90正极的横截面SEM图像; (b) NCMA90和CSG-NCMA90正极在100% DoD和60% DoD下循环的长期循环性能[85]; (b)掺铝的浓度梯度LiNi0.61Co0.12Mn0.27O2长循环性能[87]。
Figure 10. (a) Cross-sectional SEM images of the as-prepared NCM90, NCMA90, CSG-NCM90, and CSG-NCMA90 cathodes. (b) Long-term cycling performance of NCMA90 and CSG-NCMA90 cathodes cycled at 100% DoD and upper 60% DoD using pouch-type full cells [85]. (c) Long-cycle-life of Al-doped LiNi0.61Co0.12Mn0.27O2 (≥
3000 cycles with 100% depth of discharge) [87].图 11 (a)富镍多晶和单晶正极在延长循环过程中的裂纹演化和内部形态差异示意图。(b) 55℃时富镍多晶和单晶在2.75-4.4 V电压范围内的循环性能[94]。
Figure 11. (a) Schematic illustration of crack evolution and the internal morphological difference for polycrystal and single crystal. (b) Cycling performance polycrystal and single crystal Ni-rich cathode at 55℃ in the potential of 2.75-4.4 V [94].
图 12 (a)己二腈作为双功能添加剂在Li/FCG73电池中的工作示意图[97];(b) LiPF6水解生成酸性化合物及电池中活性物质(HF 和 PF5)的示意图;(c) TMS-ON对H2O和HF的清除机制[98]。
Figure 12. (a) Schematic illustration of the effectiveness of adiponitrile as a bi-functional additive in the proposed Li/FCG73 battery [97]. (b) Schematic illustration of the hydrolysis of LiPF6 derived problems in batteries. (c) HF and H2O scavenging mechanisms of TMS-ON [98].
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[1] ZHOU Xin, HONG Feifei, WANG Shou, et al. Precision engineering of high-performance Ni-rich layered cathodes with radially aligned microstructure through architectural regulation of precursors[J]. eScience, 2024, 100276.
[2] WANG Xin, ZHOU Xin, LIU Xiaohong, et al. Surface reactivity versus microcracks in Ni-rich layered oxide cathodes: Which is critical for long cycle life?[J]. Chemical Engineering Journal, 2024, 488: 150795. DOI: 10.1016/j.cej.2024.150795
[3] WANG Xin, WANG Shou, REN Li, et al. Micro-structure tuning and evolution of hydroxide precursor with radially oriented grains during industrial-scale continuous precipitation process[J]. Journal of Alloys and Compounds, 2024, 977: 173458. DOI: 10.1016/j.jallcom.2024.173458
[4] LYU Yingchun, WU Xia, WANG Kai, et al. An Overview on the Advances of LiCoO2 Cathodes for Lithium-Ion Batteries[J]. Advanced Energy Materials, 2021, 11(2): 2000982. DOI: 10.1002/aenm.202000982
[5] PADHI A K, NANJUNDASWARY K S, GOODENOUGH J B. Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries[J]. Journal of The Electrochemical Society, 1997, 144(4): 1188. DOI: 10.1149/1.1837571
[6] YIN Huaqi, LIU Wei, GU Mingzhe, et al. Li4Ti5O12 Modified LiMn2O4 Hollow Microspheres as High Rate Cathode Materials for Lithium-Ion Batteries[J]. Energy and Environment Focus, 2013, 2: 235-239. DOI: 10.1166/eef.2013.1058
[7] WANG Shou, ZHOU Xin, ZHAO Tian, et al. Precise regulation of particle orientation for Ni-rich cathodes with ultra-long cycle life[J]. Nano Energy, 2024, 129: 110008. DOI: 10.1016/j.nanoen.2024.110008
[8] 王硕, 武文斌, 王鑫, 等. 锂离子电池富镍正极基础科学问题: 径向有序多晶调控及机理[J]. 复合材料学报, 2023, 40(10): 5518-5528. WANG Shuo, WU Wenbin, WANG Xin, et al. Basic scientific problems of nickel-rich cathode for lithium-ion battery: Regulation and formation mechanism of radially oriented parties[J]. Acta Materiae Compositae Sinica, 2023, 40(10): 5518-5528(in Chinese).
[9] NOH Hyung Joo, YOUN Sungjune, YONG Chong Seung, et al. Comparison of the structural and electrochemical properties of layered Li[Ni xCo yMn z]O2 (x=1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) cathode material for lithium-ion batteries[J]. Journal of Power Sources, 2013, 233: 121-130. DOI: 10.1016/j.jpowsour.2013.01.063
[10] HUA Weibo, CHEN Jinniu, FERREIRA SANCHEZ Dario, et al. Probing Particle-Carbon/Binder Degradation Behavior in Fatigued Layered Cathode Materials through Machine Learning Aided Diffraction Tomography[J]. Angewandte Chemie International Edition, 2024, 63(30): e202403189. DOI: 10.1002/anie.202403189
[11] HUA Weibo, ZHANG Jilu, WANG Suning, et al. Long-Range Cationic Disordering Induces two Distinct Degradation Pathways in Co-Free Ni-Rich Layered Cathodes[J]. Angewandte Chemie International Edition, 2023, 62(12): e202214880. DOI: 10.1002/anie.202214880
[12] CHEN Jinniu, YANG Yang, TANG Yushu, et al. Constructing a Thin Disordered Self-Protective Layer on the LiNiO2 Primary Particles Against Oxygen Release[J]. Advanced Functional Materials, 2023, 33(6): 2211515. DOI: 10.1002/adfm.202211515
[13] ZHANG Shu, MA Jun, HU Zhenglin, et al. Identifying and Addressing Critical Challenges of High-Voltage Layered Ternary Oxide Cathode Materials[J]. Chemistry of Materials, 2019, 31(16): 6033-6065. DOI: 10.1021/acs.chemmater.9b01557
[14] CHEN Siqi, ZHOU Xin, WANG Shou, et al. High-performance single crystal Ni-rich cathode with regulated lattice and interface constructed by separated lithiation and crystallization calcination[J]. Green Chemical Engineering, 2024.
[15] CHEN Siqi, ZHANG Ping, ZHOU Xin, et al. Slightly Li-enriched chemistry enabling super stable LiNi0.5Mn0.5O2 cathodes under extreme conditions[J]. Chemical Science, 2024, 15(35): 14415-14424. DOI: 10.1039/D4SC03805C
[16] LU Jingyu, XU Chao, DOSE Wesley, et al. Microstructures of layered Ni-rich cathodes for lithium-ion batteries[J]. Chemical Society Reviews, 2024, 53(9): 4707-4740. DOI: 10.1039/D3CS00741C
[17] 曹梦圆, 王明星, 邓中莉, 等. 锂离子电池富镍正极基础科学问题: 晶粒形态及组装方式调控[J]. 复合材料学报, 2023, 40 (5): 2526-2536. CAO Mengyuan, WANG Mingxing, DENG Zhongli, et al. Basic scientific problems of nickel-rich cathode for lithium-ion battery: Morphology and accumulation regulation of primary particles[J]. Acta Materiae Compositae Sinica, 2023, 40(5): 2525-2535(in Chinese).
[18] CHEN Huan, YUAN Huihui, DAI Zhongqin, et al. Surface Gradient Ni-Rich Cathode for Li-Ion Batteries[J]. Advanced Materials, 2024, 36(33): 2401052. DOI: 10.1002/adma.202401052
[19] ZHANG Sheng S. Problems and their origins of Ni-rich layered oxide cathode materials[J]. Energy Storage Materials, 2020, 24: 247-254. DOI: 10.1016/j.ensm.2019.08.013
[20] 王鑫, 陈欣, 任莉, 等. 锂离子电池富镍正极材料基础科学问题: 表面残锂及其去除[J]. 复合材料学报, 2022, 39(1): 99-112. WANG Xin, CHEN Xin, REN Li, et al. Basic scientific problems of nickel rich cathode materials for Li-ion battery: surface residual lithium and its removal[J]. Acta Materiae Compositae Sinica, 2022, 39(1): 99-112(in Chinese).
[21] CHO Dae Hyun, JO Chang Heum, CHO Woosuk, et al. Effect of Residual Lithium Compounds on Layer Ni-Rich Li[Ni0.7Mn0.3]O2[J]. Journal of The Electrochemical Society, 2014, 161(6): 16843-16851.
[22] LIU Wen, OH Pilgun, LIU Xien, et al. Nickel-Rich Layered Lithium Transition-Metal Oxide for High-Energy Lithium-Ion Batteries[J]. Angewandte Chemie International Edition, 2015, 54(15): 4440-4457. DOI: 10.1002/anie.201409262
[23] ZHANG Wujiu, YUAN Chuhan, ZHU Jifu, et al. Air Instability of Ni-Rich Layered Oxides–A Roadblock to Large Scale Application[J] Advanced Energy Materials, 2022, 13: 2202993.
[24] PARK Jun Ho, CHOI Byungjin, KANG Yoon Sok, et al. Effect of Residual Lithium Rearrangement on Ni-rich Layered Oxide Cathodes for Lithium-Ion Batteries[J]. Energy Technology, 2018, 6(7): 1361-1369. DOI: 10.1002/ente.201700950
[25] XU Sheng, DU Chunyu, XU Xing, et al. A Mild Surface Washing Method Using Protonated Polyaniline for Ni-rich LiNi0.8Co0.1Mn0.1O2 Material of Lithium Ion Batteries[J]. Electrochimica Acta, 2017, 248: 534-540. DOI: 10.1016/j.electacta.2017.07.169
[26] XIONG Xunhui, WANG Zhixing, YUE Peng, et al. Washing effects on electrochemical performance and storage characteristics of LiNi0.8Co0.1Mn0.1O2 as cathode material for lithium-ion batteries[J]. Journal of Power Sources, 2013, 222: 318-325. DOI: 10.1016/j.jpowsour.2012.08.029
[27] ANDERSSON A M, ANRAHAN D P, HAASCH R, et al. Surface Characterization of Electrodes from High Power Lithium-Ion Batteries[J]. Journal of The Electrochemical Society, 2002, 149(10): A1358. DOI: 10.1149/1.1505636
[28] LIU Hansan, YANG Yong, ZHANG Jiujun. Investigation and improvement on the storage property of LiNi08Co0.2O2 as a cathode material for lithium-ion batteries[J]. Journal of Power Sources, 2006, 162(1): 644-650. DOI: 10.1016/j.jpowsour.2006.07.028
[29] GOODENOUGH John B, KIM Youngsik. Challenges for Rechargeable Li Batteries[J]. Chemistry of Materials, 2010, 22(3): 587-603. DOI: 10.1021/cm901452z
[30] WEI Yi, ZHENG Jiaxin, CUI Suihan, et al. Kinetics Tuning of Li-Ion Diffusion in Layered Li(Ni xMn yCo z)O2[J]. Journal of the American Chemical Society, 2015, 137(26): 8364-8367. DOI: 10.1021/jacs.5b04040
[31] DELMAS C, PERES J P, ROUGIER A, et al. On the behavior of the Li xNiO2 system: an electrochemical and structural overview[J]. Journal of Power Sources, 1997, 68: 120-125. DOI: 10.1016/S0378-7753(97)02664-5
[32] PERES J P, Delmas C, Rougier A, et al. The relationship between the composition of lithium nickel oxide and the loss of reversibility during the first cycle[J]. Journal of Physics and Chemistry of Solids, 1996, 57: 1057-1060. DOI: 10.1016/0022-3697(95)00395-9
[33] TEICHERT Philipp, ESHETU Gebrekidan Gebresilassie, JAHNKE Hannes, et al. Degradation and Aging Routes of Ni-Rich Cathode Based Li-Ion Batteries[J]. Batteries-Basel, 2020, 6(1): 8. DOI: 10.3390/batteries6010008
[34] YU Haijun, QIAN Yumin, OTANI Minoru, et al. Study of the lithium/nickel ions exchange in the layered LiNi0.42Mn0.42Co0.16O2 cathode material for lithium ion batteries: experimental and first-principles calculations[J]. Energy & Environmental Science, 2014, 7(3): 1068-1078.
[35] JOHN B. Goodenough: Magnetism and the Chemical Bond[J]. Berichte der Bunsengesellschaft für physikalische Chemie, 1964, 68(10): 996.
[36] HAUSBRAND R, CHERKASHININ G, EHRENBERG H, et al. Fundamental degradation mechanisms of layered oxide Li-ion battery cathode materials: Methodology, insights and novel approaches[J]. Materials Science and Engineering: B, 2015, 192: 3-25. DOI: 10.1016/j.mseb.2014.11.014
[37] ZHANG Xiaoyu, JIANG W J, MANGER A, et al. Minimization of the cation mixing in Li1- x(NMC)1 -xO2 as cathode material[J]. Journal of Power Sources, 2010, 195(5): 1292-1301. DOI: 10.1016/j.jpowsour.2009.09.029
[38] MYUNG Seung Taek, KOMABA Shinchi, KURIHARA Koutarou, et al. Synthesis of Li[(Ni0.5Mn0.5)1- xLi x]O2 by emulsion drying method and impact of excess Li on structural and electrochemical properties[J]. Chemistry of Materials, 2006, 18(6): 1658-1666. DOI: 10.1021/cm052704j
[39] WANG Dawei, KOU Ronghui, REN Yang, et al. Synthetic Control of Kinetic Reaction Pathway and Cationic Ordering in High-Ni Layered Oxide Cathodes[J]. Advanced Materials, 2017, 29(39): 1606715. DOI: 10.1002/adma.201606715
[40] 王鑫, 任莉, 王硕, 等. 锂离子电池富镍正极基础科学问题: 氢氧化物前驱体结晶调控及机制[J]. 复合材料学报, 2022, 39 (5): 1971-1989. WANG Xin, REN Li, WANG Shuo, et al. Basic scientific problems of nickel rich cathode materials for Li-ion battery: Regulation and mechanism for crystallization of hydroxide precursor[J]. Acta Materiae Compositae Sinica, 2022, 39(5): 1995-2013(in Chinese).
[41] DUAN Yandong, YANG Luyi, ZHANG Ming Jian, et al. Insights into Li/ Ni ordering and surface reconstruction during synthesis of Ni- rich layered oxides[J]. Journal of Materials Chemistry A, 2019, 7(2): 513-519. DOI: 10.1039/C8TA10553G
[42] 任莉, 王鑫, 王硕, 等. 锂离子电池富镍正极基础科学问题: 前驱体高温锂化过程结构演变及调控[J]. 稀有金属材料与工程, 2024, 53(5): 1493-1502. DOI: 10.12442/j.issn.1002-185X.20230137 REN li, WANG xin, WANG shuo, et al. Basic Scientific Problems of Nickel-Rich Cathode for Lithium-Ion Battery: Structural Evolution and Regulation of Intermediate During High Temperature Lithiation Process[J]. Acta Materiae Compositae Sinica, 2024, 53(5): 1493-1502(in Chinese). DOI: 10.12442/j.issn.1002-185X.20230137
[43] TIAN Xiaolu, YI Yikun, FANG Binran, et al. Design Strategies of Safe Electrolytes for Preventing Thermal Runaway in Lithium Ion Batteries[J]. Chemistry of Materials, 2020, 32: 9821-9848. DOI: 10.1021/acs.chemmater.0c02428
[44] STREICH Daniel, ERK Chrisstoph, GUEGUEN Aurelie, et al. Operando Monitoring of Early Ni-mediated Surface Reconstruction in Layered Lithiated Ni-Co-Mn Oxides[J]. Journal of Physical Chemistry C, 2017, 121(25): 13481-13486. DOI: 10.1021/acs.jpcc.7b02303
[45] LIN Qingyun, GUAN Wenhao, MENG Jie, et al. A new insight into continuous performance decay mechanism of Ni-rich layered oxide cathode for high energy lithium ion batteries[J]. Nano Energy, 2018, 54: 313-321. DOI: 10.1016/j.nanoen.2018.09.066
[46] GUILMARD M, CROGUENNEC L, DENUX D, et al. Thermal stability of lithium nickel oxide derivatives. Part I: Li xNi1.02O2 and Li xNi0.89Al0.16O2 (x=0.50 and 0.30)[J]. Chemistry of Materials, 2003, 15(23): 4476-4483. DOI: 10.1021/cm030059f
[47] WU Lijun, NAM Kyung Wan, WANG Xiaojian, et al. Structural Origin of Overcharge-Induced Thermal Instability of Ni-Containing Layered-Cathodes for High-Energy-Density Lithium Batteries[J]. Chemistry of Materials, 2011, 23(17): 3953-3960. DOI: 10.1021/cm201452q
[48] ZHANG Sheng S. Understanding of performance degradation of LiNi0.80Co0.10Mn0.10O2 cathode material operating at high potentials[J]. Journal of Energy Chemistry, 2020, 41: 135-141. DOI: 10.1016/j.jechem.2019.05.013
[49] KANG Yoon Sok, PARK Seong Yong, ITO Kimihiko, et al. Revealing the structural degradation mechanism of the Ni-rich cathode surface: How thick is the surface?[J]. Journal of Power Sources, 2021, 490: 229542. DOI: 10.1016/j.jpowsour.2021.229542
[50] SUN Ho Hyun, MANTHIRAM Arumugan. Impact of Microcrack Generation and Surface Degradation on a Nickel-Rich Layered Li[Ni0.9Co0.05Mn0.05]O2 Cathode for Lithium-Ion Batteries[J]. Chemistry of Materials, 2017, 29(19): 8486-8493. DOI: 10.1021/acs.chemmater.7b03268
[51] XU Chao, MARKER Katharina, LEE Juhan, et al. Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries[J]. Nature Materials, 2021, 20(1): 84-92. DOI: 10.1038/s41563-020-0767-8
[52] LIN Qingyun, GUAN Wenhao, ZHOU Jianbin, et al. Ni-Li anti-site defect induced intragranular cracking in Ni-rich layer-structured cathode[J]. Nano Energy, 2020, 76: 105021. DOI: 10.1016/j.nanoen.2020.105021
[53] MIN Kyoungmin, CHO Eunseog. Intrinsic origin of intra-granular cracking in Ni-rich layered oxide cathode materials[J]. Physical Chemistry Chemical Physics, 2018, 20(14): 9045-9052. DOI: 10.1039/C7CP06615E
[54] YIN Shouyi, DENG Wentao, CHEN Jun, et al. Fundamental and solutions of microcrack in Ni-rich layered oxide cathode materials of lithium-ion batteries[J]. Nano Energy, 2021, 83: 105854. DOI: 10.1016/j.nanoen.2021.105854
[55] KIM Jae Hyung, PARK Kang Joon, KIM Suk Jun, et al. A method of increasing the energy density of layered Ni-rich Li[Ni1-2 xCo xMn x]O2 cathodes (x=0.05, 0.1, 0.2)[J]. Journal of Materials Chemistry A, 2019, 7(6): 2694-2701. DOI: 10.1039/C8TA10438G
[56] YAN Pengfei, ZHENG Jianmin, GU Meng, et al. Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries[J]. Nature Communications, 2017, 8: 14101. DOI: 10.1038/ncomms14101
[57] NAM Gyeong Won, PARK Nam Yung, PARK Kang-Joon, et al. Capacity Fading of Ni-Rich NCA Cathodes: Effect of Microcracking Extent[J]. ACS Energy Letters, 2019, 4(12): 2995-3001. DOI: 10.1021/acsenergylett.9b02302
[58] SHI Yang, ZHANG Minghao, QIAN Danna, et al. Ultrathin Al2O3 Coatings for Improved Cycling Performance and Thermal Stability of LiNi0.5Co0.2Mn0.3O2 Cathode Material[J]. Electrochimica Acta, 2016, 203: 154-161. DOI: 10.1016/j.electacta.2016.03.185
[59] FENG Yanhua, ZHANG Xiangxin, LIN Changxin, et al. Improving the electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathodes using a simple Ce4+-doping and CeO2-coating technique[J]. New Journal of Chemistry, 2021, 45(46): 21617-21623. DOI: 10.1039/D1NJ04997F
[60] MA Yuan, TEO Jun Hao, WALTHER Felix, et al. Advanced Nanoparticle Coatings for Stabilizing Layered Ni-Rich Oxide Cathodes in Solid-State Batteries[J]. Advanced Functional Materials, 2022, 32(23): 2111829. DOI: 10.1002/adfm.202111829
[61] CHEN Yanping, ZHANG Yun, CHEN Baojun, et al. An approach to application for LiNi0.6Co0.2Mn0.2O2 cathode material at high cutoff voltage by TiO2 coating[J]. Journal of Power Sources, 2014, 256: 20-27. DOI: 10.1016/j.jpowsour.2014.01.061
[62] LIU Kai, ZHANG Qingqing, DAI Sheng, et al. Synergistic Effect of F- Doping and LiF Coating on Improving the High-Voltage Cycling Stability and Rate Capacity of LiNi0.5Co0.2Mn0.3O2 Cathode Materials for Lithium-Ion Batteries[J]. ACS Applied Materials & Interfaces, 2018, 10(40): 34153-34162.
[63] LEE Sang Hyuk, YOON Chong Seung, AMINE Khalil, et al. Improvement of long-term cycling performance of Li[Ni0.8Co0.15Al0.05]O2 by AlF3 coating[J]. Journal of Power Sources, 2013, 234: 201-207. DOI: 10.1016/j.jpowsour.2013.01.045
[64] CROMPTON K R, HLADKY M P, STAUB J W, et al. Enhanced Overdischarge Stability of LiCoO2 by a Solution Deposited AlPO4 Coating[J]. Journal of the Electrochemical Society, 2017, 164(13): A3214-A3219. DOI: 10.1149/2.1171713jes
[65] BAI Yansong, WANG Xianyou, YANG Shunyi, et al. The effects of FePO4-coating on high-voltage cycling stability and rate capability of Li[Ni0.5Co0.2Mn0.3]O2[J]. Journal of Alloys and Compounds, 2012, 541: 125-131. DOI: 10.1016/j.jallcom.2012.06.101
[66] HAN Yongkang, HENG Shuai, WANG Yan, et al. Anchoring Interfacial Nickel Cations on Single-Crystal LiNi0.8Co0.1Mn0.1O2 Cathode Surface via Controllable Electron Transfer[J]. ACS Energy Letters, 2020, 5(7): 2421-2433. DOI: 10.1021/acsenergylett.0c01032
[67] SUN Qian, HU Guorong, PENG Zhongdong, et al. Achieving a bifunctional conformal coating on nickel-rich cathode LiNi0.8Co0.1Mn0.1O2 with half-cyclized polyacrylonitrile[J]. Electrochimica Acta, 2021, 386: 138440. DOI: 10.1016/j.electacta.2021.138440
[68] YAN Pengfei, ZHENG Jianming, LIU Jian, et al. Tailoring grain boundary structures and chemistry of Ni-rich layered cathodes for enhanced cycle stability of lithium-ion batteries[J]. Nature Energy, 2018, 3(7): 600-605. DOI: 10.1038/s41560-018-0191-3
[69] XU Ya Di, XIANG Wei, WU Zhen Guo, et al. Improving cycling performance and rate capability of Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode materials by Li4Ti5O12 coating[J]. Electrochimica Acta, 2018, 268: 358-365. DOI: 10.1016/j.electacta.2018.02.049
[70] XU Guiliang, LIU Qiang, LAU KENNETH K S, et al. Building ultraconformal protective layers on both secondary and primary particles of layered lithium transition metal oxide cathodes[J]. Nature Energy, 2019, 4: 484-494. DOI: 10.1038/s41560-019-0387-1
[71] KIM Un Hyuck, PARK Geon Tae, SON Byoung Ki, et al. Heuristic solution for achieving long-term cycle stability for Ni-rich layered cathodes at full depth of discharge[J]. Nature Energy, 2020, 5(11): 860-869. DOI: 10.1038/s41560-020-00693-6
[72] SUN H Hohyun, DOLOCAN Andrei, WEEKS Jason A. , et al Stabilization of a Highly Ni-Rich Layered Oxide Cathode through Flower-Petal Grain Arrays[J]. ACS Nano, 2020, 14(12): 17142-17150. DOI: 10.1021/acsnano.0c06910
[73] XIE Qiang, LI Wangda, MANTHIRAM Arumugam. A Mg-Doped High-Nickel Layered Oxide Cathode Enabling Safer, High-Energy-Density Li-Ion Batteries[J]. Chemistry of Materials, 2019, 31(3): 938-946. DOI: 10.1021/acs.chemmater.8b03900
[74] PARK Kang Joon, JUNG Hun Gi, KUO Liang Yin, et al. Improved Cycling Stability of Li[Ni0.90Co0.05Mn0.05]O2 Through Microstructure Modification by Boron Doping for Li-Ion Batteries[J]. Advanced Energy Materials, 2018, 8(25): 1801202. DOI: 10.1002/aenm.201801202
[75] CHENG Lei, ZHANG Bao, SU Shi Lin, et al. Al-doping enables high stability of single-crystalline LiNi0.7Co0.1Mn0.2O2 lithium-ion cathodes at high voltage[J]. RSC Advances, 2021, 11(1): 124-128. DOI: 10.1039/D0RA09813B
[76] HUA Wei, ZHANG Jibin, ZHENG Zhuo, et al. Na-doped Ni-rich LiNi0.5Co0.2Mn0.3O2 cathode material with both high rate capability and high tap density for lithium ion batteries[J]. Dalton Trasactions, 2014, 43(39): 14824-14832. DOI: 10.1039/C4DT01611D
[77] LEE Soo Been, PARK Nam Yung, KIM Un Hyuck, et al. Doping Strategy in Developing Ni-Rich Cathodes for High-Performance Lithium-Ion Batteries[J]. ACS Energy Letters, 2024, 9(2): 740-747. DOI: 10.1021/acsenergylett.3c02759
[78] JING Zhiwei, WANG Suning, FU Qiang, et al. Architecting “Li-rich Ni-rich” core-shell layered cathodes for high-energy Li-ion batteries[J]. Energy Storage Materials, 2023, 59: 102775. DOI: 10.1016/j.ensm.2023.102775
[79] WANG Liguang, LEI Xincheng, LIU Tongchao, et al. Regulation of Surface Defect Chemistry toward Stable Ni-Rich Cathodes[J]. Advanced Materials, 2022, 34(19): 2200744. DOI: 10.1002/adma.202200744
[80] SUN Yang Kook, MYUNG Seung Taek, KIM Myung Hoon, et al. Synthesis and characterization of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 with the microscale core-shell structure as the positive electrode material for lithium batteries[J]. Journal of the American Chemical Society, 2005, 127(38): 13411-13418. DOI: 10.1021/ja053675g
[81] SUN Yang Kook, MYUNG Seung Taek, PARK Byung Chun, et al. High-energy cathode material for long-life and safe lithium batteries[J]. Nature Materials, 2009, 8(4): 320-324. DOI: 10.1038/nmat2418
[82] KIM Un Hyuck, RYU Hoon Hee, KIM Jae Hyung, et al. Microstructure-Controlled Ni-Rich Cathode Material by Microscale Compositional Partition for Next-Generation Electric Vehicles[J]. Advanced Energy Materials, 2019, 9(15): 1803902. DOI: 10.1002/aenm.201803902
[83] SUN Yang Kook, CHEN Zonghai, NOH Hyung Joo, et al. Nanostructured high-energy cathode materials for advanced lithium batteries[J]. Nature Materials, 2012, 11(11): 942-947. DOI: 10.1038/nmat3435
[84] LIM Byung Beom, YOON Sung Jun, PARK Kang Joon, et al. Advanced Concentration Gradient Cathode Material with Two-Slope for High-Energy and Safe Lithium Batteries[J]. Advanced Functional Materials, 2015, 25(29): 4673-4680. DOI: 10.1002/adfm.201501430
[85] PARK Nam Yung, RYU Hoon Hee, PARK Geon Tae, et al. Optimized Ni-Rich NCMA Cathode for Electric Vehicle Batteries[J]. Advanced Energy Materials, 2021, 11(9): 2003767. DOI: 10.1002/aenm.202003767
[86] LI Wangda, SONG Bohang, MANTHIRAM Arumugam. High-voltage positive electrode materials for lithium-ion batteries[J]. Chemical Society Reviews, 2017, 46(10): 3006-3059. DOI: 10.1039/C6CS00875E
[87] KIM Un Hyuck, LEE Eung Ju, YOON Chong S. et al. Compositionally Graded Cathode Material with Long-Term Cycling Stability for Electric Vehicles Application[J]. Advanced Energy Materials, 2016, 6(22).
[88] YAN Yawen, FANG Qiu, KUAI Xiaoxiao, et al. One-Step Surface-to-Bulk Modification of High-Voltage and Long-Life LiCoO2 Cathode with Concentration Gradient Architecture[J]. Advanced Materials, 2023, 36(1): e2308656.
[89] MENG Junxia, XU Lishuang, MA Quanxin, et al. Modulating Crystal and Interfacial Properties by W-Gradient Doping for Highly Stable and Long Life Li-Rich Layered Cathodes[J]. Advanced Functional Materials, 2022, 32(19): 2113013. DOI: 10.1002/adfm.202113013
[90] PARK Geon Tae, NAMKOONG Been, KIM Su Bin, et al. Introducing high-valence elements into cobalt-free layered cathodes for practical lithium-ion batteries[J]. Nature Energy, 2022, 7(10): 946-954. DOI: 10.1038/s41560-022-01106-6
[91] WANG Ruijuan, CHEN Jiarui, ZHANG Yixu, et al. Inhibiting phase conversion and improving cyclic stability of Ni-rich layered oxide by high-valence element concentration gradient doping[J]. Chemical Engineering Journal, 2024, 485: 149827. DOI: 10.1016/j.cej.2024.149827
[92] WANG Yinzhong, WANG Errui, ZHANG Xu, et al. High-Voltage “Single-Crystal” Cathode Materials for Lithium-Ion Batteries[J]. Energy & Fuels, 2021, 35(3): 1918-1932.
[93] QIAN Guannan, ZHANG Youtian, LI Linsen, et al. Single-crystal nickel-rich layered-oxide battery cathode materials: synthesis, electrochemistry, and intra-granular fracture[J]. Energy Storage Materials, 2020, 27: 140-149. DOI: 10.1016/j.ensm.2020.01.027
[94] FAN Xinming, HU Guorong, ZHANG Bao, et al. Crack-free single-crystalline Ni-rich layered NCM cathode enable superior cycling performance of lithium-ion batteries[J]. Nano Energy, 2020, 70: 104450. DOI: 10.1016/j.nanoen.2020.104450
[95] ZHAO Wengao, ZOU Lianfeng, ZHENG Jianming, et al. Simultaneous Stabilization of LiNi0.76Mn0.14Co0.10O2 Cathode and Lithium Metal Anode by Lithium Bis(oxalato)borate as Additive[J]. ChemSusChem, 2018, 11(13): 2211-2220. DOI: 10.1002/cssc.201800706
[96] LI Jianhui, LIAO Yyuqing, FAN Weizhen, et al. Significance of Electrolyte Additive Molecule Structure in Constructing Robust Interphases on High-Voltage Cathodes[J]. ACS Applied Energy Materials, 2020, 3(3): 3049-3058. DOI: 10.1021/acsaem.0c00168
[97] LEE Seon Hwa, HWANG Jang Yeon, PARK Seong Jin, et al. Adiponitrile (C6H8N2): A New Bi-Functional Additive for High-Performance Li-Metal Batteries[J]. Advanced Functional Materials, 2019, 29(30): 1902496. DOI: 10.1002/adfm.201902496
[98] KIM Koeun, HWANG Daeyeon, KIM Saehun, et al. Cyclic Aminosilane-Based Additive Ensuring Stable Electrode–Electrolyte Interfaces in Li-Ion Batteries. Advanced Energy Materials 2020, 10: 2000012.
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目的
锂离子电池(LIBs)作为能源存储领域的重要组成部分,尤其在电动汽车和电网储能系统中的应用,引发了广泛的研究关注。富镍正极材料凭借其高容量和成本效益,已成为提升锂电池能量密度的关键方向。然而,随着镍含量的增加,该类材料在稳定性方面表现出诸多问题,如晶体结构失稳、表面残锂、阳离子混排、气体释放及不可逆相变等。这些问题严重限制了电池的循环寿命和安全性。基于此,本文旨在系统分析富镍正极材料失稳的机制,并探讨通过改性手段,如表面涂层、元素掺杂和电解质添加剂等,提升材料稳定性的有效策略,从而为未来锂电池正极材料的优化设计提供理论依据和技术支持。
方法本文采用系统性文献回顾的方式,对富镍正极材料的失稳机制及其改性方法进行了深入分析。首先,从材料结构的角度探讨富镍正极在高镍含量下的失稳特征,特别是表面残锂化合物的生成、阳离子混排及不可逆相变的发生机制。其次,归纳总结了当前文献中提出的主要改性策略,包括表面涂层技术、元素掺杂技术、单晶化以及浓度梯度结构设计。同时,本文还探讨了电解质添加剂在改善材料电化学性能中的作用,重点分析其通过抑制电极与电解质副反应以提升材料长循环性能的潜力。
结果本研究对富镍正极材料的失稳机制和提升稳定性的改性策略进行了系统分析与总结。结果显示,富镍正极材料的失稳主要与以下几个因素有关:表面残锂化合物、阳离子混排现象、气体释放、不可逆的相转变、以及微裂纹的形成。其中,残锂化合物会导致材料表面化学稳定性变差,影响电极制造过程;阳离子混排则会阻碍锂离子的扩散动力学,降低材料的倍率性能和循环寿命;气体释放源于晶格氧损失及电解液副反应,不仅影响材料的电化学性能,还对电池安全性构成威胁;不可逆相变和微裂纹的产生会削弱材料的机械稳定性,进而加剧电池容量的衰减。针对这些失稳因素,研究总结了几种有效的改性策略。首先,元素掺杂能够有效减少阳离子混排,增强材料的结构稳定性;其次,表面包覆通过在材料表面形成保护层,抑制电解液的侵蚀和气体释放,提高了电池的循环稳定性;单晶化策略减少了晶界应力,降低了微裂纹的产生概率,从而提升了电池的长循环性能;浓度梯度结构设计则有效地优化了富镍材料的表面与内部组成,兼顾了高容量和长循环稳定性;最后,通过使用电解质添加剂,可以在材料表面形成稳定的固体电解质界面膜(CEI膜),从而抑制副反应、减少气体释放,进一步提高材料的电化学性能。
结论富镍正极材料由于其较高的能量密度和优异的倍率性能,已成为锂离子电池正极材料的研究重点。然而,随着镍含量的提升,材料在循环过程中的化学稳定性和结构稳定性明显下降,进而影响其长期使用性能与安全性。通过表面涂层、元素掺杂、单晶化和浓度梯度结构设计等一系列改性策略,可以在不同程度上提升材料的电化学性能和循环寿命。未来的研究应聚焦于多重改性策略的协同机制,进一步优化组合不同改性方法的效果,最大限度发挥其综合性能。同时,利用先进的表征技术深入剖析材料在不同使用条件下的失效机制,将为富镍正极材料的稳定性提升提供全新的理论支持。