Enhancement in the electrochemical performance of zirconium/phosphate bi-functional coatings on LiNi_0.8Co_0.15Mn_0.05O₂ by the removal of Li residuals

Abstract

The effect of bi-functional coatings consisting of Zr and phosphate (P) on the electrochemical performance of LiNiCoMnO (NCM) has been investigated. The presence of various types of Zr and P compounds such as oxides (ZrO and LiZrO) and phosphates (ZrPO, ZrPO and LiZr(PO)) in the coating was confirmed by experiments as well as density functional theory (DFT) calculations. When the NCM samples were coated with the Zr/P hybrid material, the cycle retention and the amount of removed Li residuals (LiOH, LiCO) were enhanced by the synergistic effect from Zr and P. The NCM sample coated with a Zr/P layer with a Zr/P ratio of 1 : 1 exhibited an increase in the initial capacity (209.3 mA h g) compared to the pristine sample (207.4 mA h g) at 0.1C, owing to the formation of the coating layer. The capacity retention of the Zr/P coated sample (92.4% at the 50th cycle) was also improved compared to that of the pristine NCM sample (90.6% at the 50th cycle). Moreover, the amount of Li residuals in the Zr/P coated NCM sample was greatly reduced from 3693 ppm (pristine NCM) to 2525 ppm (Zr/P = 5 : 5).

Improved electrochemical properties of LiNi0.91Co0.06Mn0.03O2 cathode material via Li-reactive coating with metal phosphates

“…To mitigate degradation behaviors in layered oxide cathode materials thus enhancing the electrochemical performance, the surface modification method, i.e ., surface coating, has been suggested to provide a physical barrier at the surface of the cathode and prevent the direct contact between active materials and electrolytes. For example, many metal phosphate (MP) materials are suggested as effective coating materials such as MPO 4 (M = Al, Fe, Ce, and Sr) 8 , 9 , Ni 3 (PO 4 ) 2 10 , Mn 3 (PO 4 ) 2 11 , M 3 (PO 4 ) 2 (M = Zn and Mg) 12 , and Zr-phosphate 13 for cathode materials such as LiCoO 2 (LCO), LiNi 0.9 Co 0.1 O 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA), LiNi 0.6 Co 0.2 Mn 0.2 O 2 , and LiNi 0.8 Co 0.15 Mn 0.05 O 2 . In spite of their effectiveness in improving the capacity retention rate, this type of coating approach in principle has drawbacks: 1) Li ion diffusion during electrochemical cycling can be impeded, 2) residual Li still needs to be washed, which requires an additional step during synthesis, and 3) this process can degrade battery performance 14 – 16 .…”

Research Progress on Coating and Doping Modification of Nickel Rich Ternary Cathode Materials

“…高镍三元正极材料循环性能不佳, 除了副反应 之外, 还与充放电过程中的表面结构变化和二次颗 粒破碎有关 [27,[67][68] 。某些特定的包覆剂在隔绝电解 液、减少副反应的同时, 也能起到稳定表面结构和 抑制微裂纹的作用。如硅酸盐和磷酸盐, 较强的 Si=O、P=O 键使过渡金属与 SiO 3 、PO 4 聚阴离子间 的共价键更强; 同时, 硅酸盐/磷酸盐比金属氧化物 具有更高的热力学与电化学稳定性, 因此能够提升 正极材料结构稳定性, 提升高电压或高温性能。 Li 2 SiO 3 包覆层能有效地隔绝高电压下电极-电解液 界面上的副反应, 更重要的是, 当锂离子在高压下 发生深度脱层时, 包覆层增强了正极材料的结构稳 定性 [69] 。此外, Li 2 SiO 3 还可以增强锂离子在电极/ 电解质界面的扩散, 防止正极材料颗粒在循环过程 中的粉化 [70] 。 Li 3 PO 4 在提升材料的倍率性能的同时, 又可以阻止 HF 和 POF 3 的侵蚀, 提升材料的循环性 能 [71] 。有证据表明, Li 3 PO 4 还能够减缓高温下的相 转变, 正极材料的初始热分解温度和放热量均明显 下降, 相转变温度提升, 并且可以抑制尖晶石和岩 盐结构之间的相转变。LiFePO 4 (LFP)正极材料得益 于其无机性质和化学稳定性, 具有优异的循环性能, 作为包覆剂使用时能够提升三元材料的高温循环稳 定性, 同时又不会降低首周放电比容量 [72] 。但是, LFP 的导电性较差, 需要通过工艺严格控制颗粒大 小和包覆层厚度 [73] 。为尽量提高三元材料的放电容 量, 需要提升其充放电截止电压, 但高电压下副反 应和不可逆相变加剧, 影响循环性能和安全性能。 在以 LFP 包覆的 NCM523 为正极组装的 5 Ah 软包 电池中, LFP 包覆层抑制了层状-尖晶石-岩盐相 的相转变, 因此该电池能够耐受 4.5 V 的高电压, 且 在满电状态下不会发生热失控 [74][75] , 其他磷酸盐也 有类似报道 [76][77] 。自从索尼公司首次推出 LiCoO 2 /C 二次电池以来, LiCoO 2 作为正极材料取得了巨大的 成功。有鉴于此, 研究者将 LiCoO 2 用于三元材料的 包覆, 发现其可以减少三元材料表面的 NiO 结构, 提升电化学性能 [78][79] 。此外, 由于 LiCoO 2 的结构稳 定性, 同时还能提升三元材料在空气中的存储性能, 减少表面镍价态、水分、残碱含量以及降低电化学 性能受暴露时间的影响 [80] 。 除材料本身的结构稳定性之外, 循环过程中二 次颗粒内部产生的微裂纹也是导致正极材料循环性 能下降的原因之一: 产生微裂纹后, 电解液即会浸 入, 并在颗粒内发生不可逆分解, 分解产物沿晶间 裂纹的积累加速了容量衰减(图 5) [81] 。在 Li 2 SiO 3 包 覆的 NCM523 中, 富硅区不仅集中在颗粒的表面, 在晶界处、 甚至在 400 nm 深处也有 Si 元素分布 [82] 。 富硅区抑制了裂纹的形成, 阻止了电解质溶液在颗 粒内的侵入和分解。LBO(硼酸锂)材料具有良好的 离子电导率和高氧化稳定性, 更重要的是在提升三 图 5 100 周循环(4.7 V)后正极材料颗粒内部微裂纹的 (a~d)SEM 照片及其(e)形成示意图 [81] Fig. 5 (a-d) SEM images of the cathode cracks in the particles cycled 100 times (4.7 V), and (e) schematic diagram showing crack formation [81] 元材料的高电压性能的同时 [83] , 也能够抑制循环过 程中微裂纹的产生, 将 NCM811 在 4.6 V 循环时微 裂纹的出现周数由 106 周延长至 211 周 [84] [54,[69][70][71]85] 。Zr 对提 升循环保持率有益处, 但对初始容量和残碱作用不 明显, P 则相反。使用 Zr/P 双包覆, 借助 Zr 和 P 的 协同作用, 可以同时提升样品的初始容量和循环性 能, 残碱的降低也较为可观 [86] 。包覆剂中的高价态…”

Thermochemical investigation of Zr doping in LiNi8/12Co2/12Mn2/12O2 based on phase equilibria simulation

“…To compensate for these problems, methods such as the doping of foreign elements to improve the structural stability of crystals or coating surfaces with other materials to suppress side reactions at the cathode‐electrolyte interface have been widely attempted . Zr is considered an effective doping and/or coating material by many researchers . Improvements in the cycle and rate properties of cathodes through Zr doping have been reported and attributed to the so‐called pillar effect: Metals with higher oxidation numbers than Li are located in the Li layer by Zr doping, reducing the repulsive interactions between O 2− ions around the Li layer.…”

“…This helps to sustain the structural stability of the cathode crystal and achieve smooth Li migration during charge and discharge of the battery . Because Zr forms highly chemically stable oxides, it has also been studied as a surface treatment material that has improved cyclic performance through the suppression of side reactions …”