This study compares the physico- and electro- chemical properties of LiNi0.8Mn0.10Co0.1O2 (NMC811) and LiNi0.83Mn0.06Co0.09Al0.1O2 (NMCA) prepared by an oxalic acid co-precipitation. Deposition of a SiO2 surface coating was attempted via reaction of the powder with an amino silane prior to the final heat treatment. It was found that either the presence of small amounts of Al3+, or the compositional gradient resulting from a two step co-precipitation, caused increased crystal growth of the NMCA in comparison to NMC811. This led to improved cyclability in LP40 electrolyte. However, the SiO2 coating appeared incomplete and negatively impacted performance. Crystal cleavage preferably on the {001} planes was observed after 100 charge-discharge cycles, with consequent cathode electrolyte interphase formation in the crystal cracks. This is believed to cause capacity decay via lithium loss, and increased charge transfer resistance. An FEC based electrolyte improved the cyclability in all cases and even under extreme conditions (45°C and upper cycling potential of 4.5 V) NMCA showed a capacity retention of 85% after 100 cycles.

In this work we report variations of LiNi 0.88 Mn 0.06 Co 0.06 O 2 synthesised through a single-pot oxalic acid co-precipitation route, in which all cation precursors were added in the same step. The effects of Al-doping, heat-treatment temperature and Li precursor excess were investigated with physicochemical and electrochemical characterisation. Phase pure and well-ordered polycrystalline materials were successfully synthesised for all Aldoped and undoped compositions. Undoped LiNi 0.88 Mn 0.06 Co 0.06 O 2 prepared at 750 °C with 4 at% excess Li precursor showed excellent cycling stability in NMC j j LTO cells with an initial capacity of 201 mAh/g at 0.1 C at 20 °C, and a capacity retention of 81 % after 415 cycles. The Al-doped variations LiNi 0.88 Mn 0.04 Co 0.06 Al 0.02 O 2 and LiNi 0.88 Mn 0.06 Co 0.04 Al 0.02 O 2 were synthesised, and they showed similar initial electrochemical performance to undoped LiNi 0.88 Mn 0.06 Co 0.06 O 2 , but Aldoping via the oxalic acid co-precipitation route resulted in shorter cycle life. The study outlines the importance of the processing parameters to achieve Ni-rich layered oxides with a long cycle life without further surface modifications.

Batteries with high energy densities and long cycle lives are important for enabling the transition from fossil fuels to electrical vehicles. Lithium ion batteries are the most popular batteries for use in electrical vehicles, but there is still a need to increase the gravimetric and volumetric energy densities from state-of-the-art batteries without increasing the costs. Since cobalt is one of the most costly and also toxic elements used in the cathode, developing low cobalt chemistries is highly desirable. LiNixMnyCozO2 (x+y+z = 1; NMC) with high Ni content is one of the cathode materials which has received attention because its high capacity and energy density. The capacity of NMC increases with increasing Ni content because Ni is the main electrochemically active transition metal in NMC. However, the capacity retention and thermal stability decreases with increasing Ni content as well [1]. Still, there is research on stabilising NMC with high Ni content, such as LiNi0.8Mn0.1Co0.1O2 (NMC 811), both with respect to performance and safety. An example of such an effort is to coat NMC 811 with AlPO4 [2]. NMC has a layered structure with lithium ions and transition metal ions in separate layers. One of several reasons for the capacity fade of NMC during cycling is cation mixing, where some Ni2+ and Li+ switch positions in the structure because of their similar ionic radii, blocking pathways for Li+ during lithiation and delithiation [3, 4, 5]. Work performed by Huaquan Lu et al. (2013) on NMC 811 indicates that the synthesis method may affect both morphology and degree of cation mixing [6]. In this work, the effect of different experimental parameters on quality of the synthesised material is studied. NMC 811 was synthesised by an oxalate precipitation from transition metal acetates and lithium nitrate precursors, adapted from a method described by Zhen Chen et al. [7]. Initial results show a phase pure layered structure with low cation mixing. Further work will include adjusting different experimental parameters, such as precursor mixing sequences, precursor mixing rates, annealing temperatures and atmosphere, in an effort to optimise the synthesis. The success of the synthesis will be evaluated based on factors such as phase purity, cation mixing, and electrochemical performance. References [1] Noh, Hyung-Joo, et al. "Comparison of the structural and electrochemical properties of layered Li[NixCoyMnz]O2 (x= 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) cathode material for lithium-ion batteries." Journal of power sources 233 (2013): 121-130. [2] Cho, Jaephil, Hyemin Kim, and Byungwoo Park. "Comparison of Overcharge Behavior of AlPO4-Coated LiCoO2 and LiNi0.8Co0.1Mn0.1O2 Cathode Materials in Li-Ion Cells." Journal of The Electrochemical Society 151.10 (2004): A1707-A1711. [3] Yu, Haijun, 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." Energy & Environmental Science 7.3 (2014): 1068-1078. [4] Wu, Feng, et al. "Effect of Ni2+ content on lithium/nickel disorder for Ni-rich cathode materials." ACS applied materials & interfaces 7.14 (2015): 7702-7708. [5] MacNeil, D. D., Z. Lu, and Jeff R. Dahn. "Structure and Electrochemistry of Li[NixCo1−2xMnx]O2 (0⩽ x⩽ 1/2)." Journal of The Electrochemical Society 149.10 (2002): A1332-A1336. [6] Lu, Huaquan, et al. "High capacity Li [Ni0.8Co0.1Mn0.1] O2 synthesized by sol–gel and co-precipitation methods as cathode materials for lithium-ion batteries." Solid State Ionics 249 (2013): 105-111. [7] Chen, Zhen, et al. "Manganese phosphate coated Li [Ni0.6Co0.2Mn0.2]O2 cathode material: Towards superior cycling stability at elevated temperature and high voltage." Journal of Power Sources 402 (2018): 263-271.