Design of boron doped (nickel manganese cobalt containing) NMC 811 cathode active materials

Abstract

Many countries have announced that they will gradually ban the sale of internal combustion engine vehicles (ICE) between 2025 and 2050 under the zero emission policy, and only zero emission vehicles (ZEV) will be sold in the near future. Nowadays, there are many brands that produce electric vehicles (EV) and they are constantly making new investments. In general, steps are being taken to improve the batteries of electric vehicles, whose biggest problem is range. According to 2023 data, lithium ion batteries have a market volume of $55 billion, and many researchers expect a compound annual growth rate (CAGR) of around 20% by 2032. Although lithium had been utilized previously in 1980, Goodenough was the first to employ a transition metal as a cathode active material in a layered (2D) structure, LiCoO2 (LCO). Then, because LCO batteries did not function at high charging rates and had a safety issue at high temperatures, due to the close ionic diameter of the Ni2+ ion and the Li+ ion, they were replaced by LiNiO2 (LNO) chemistry. When lithium leaves the cathode during charging, the Ni2+ ion fills the lithium gaps, closing the passageway of lithium. It made diffusion difficult and caused loss of capacity in the battery. Later, LiMnO2 (LMO) replaced LCO as it was economically convenient and environmentally friendly. LMO, which preserved its structure well especially at high temperatures, experienced capacity loss as a result of long cycles. Then, alternatively LiFePO4 (LFP) was utilized as cathode active material due to its environmentally friendly behavior, and high electrochemical stability at 3.5V. Later, while the specific capacity was increased by doping Ni into the LCOs, Al is added into the structure to stabilize it. Ni0.8Co0.15Al0.05 (NCA) structure offers high electrochemical performance, however, its use is restricted in some places due to security problems. NMC contains nickel, manganese and cobalt. It has been studied extensively because it offers high energy and power densities. NMC cathodes were produced in different compositions such as NMC333, NMC532, NMC622, NMC811 to optimize the capacity and the cycle life. The studies reveal that increasing Ni content in chemistry, increases the capacity of the cell but causes several problems such as chemical instability in the structures hence weak capacity retention over cycles. Today, while NMC cathode active materials can be produced with many techniques, co-precipitation method stands forward as it enables to fabricate particles with narrow particle-size distribution, high tap density with spherical morphology. The process consists of two steps: precipitation and calcination. The morphology, the structure and size of the powders obtained from the co-precipitation method are greatly affected by precipitation (pH, mixing temperature, ions concentration, mixing duration and environment) as well as calcination Therefore, in order to fabricate 5-15 micrometer sized, spherical shaped NMC811 powders via coprecipitation researchers have realized many optimization studies in the past. NMC811 suffers from low initial coulombic efficiency and capacity retention over long cycling. Structural, morphological and chemical analyses reveal that 'cation mixing', phase transformation in cycling, microcracking and hence oxygen evolution from the structure are the main problems encountered in the use of NMC811. Cation mixing is the electrochemical transformation of the crystal structure from the layered state to the rock-salt phase during the operation of the battery. Cation mixing occurs when the low-valence metal ions (Ni2+) migrate to the Li+ ion layer and replace the Li+ ions. Due to the low difference between the ionic diameters of Ni2+ (0.69Å) and Li+ (0.76Å) ions diameters among Ni2+, Co2+ and Mn2+ ions, the probability of Li+ ions being in the cation mixing with Ni2+ is higher than others. Cation mixing is not only formed during the synthesis of the material but can also be formed during the use of the battery, by redox reactions. Cation mixing (Ni2+/Li+) causes the system to be unstable thermodynamically and Ni reaches Ni2+ from high valence to low valence, causing Li and O to separate from the system, resulting in the loss of these elements and ultimately performance losses in the battery. The oxygen release from the structure can lead to safety problems since organic electrolyte systems are generally used. Structural analysis shows that NMC811 follows various phase transformation in cycling: from hexagonal to monoclinic (H1-M), from monoclinic to hexagonal (M-H2) and from hexagonal to a hexagonal structure with different lattice parameters (H2-H3). Electrochemically, the transformation from H2 to H3 phase between 4.15-4.2 V causes shrinkage around 3.7% in the c-direction in the hexagonal lattice and as a result, mechanical strain in the cathode active material, this strain causes micro cracks in the structure and leads to a decrease in cycle performance. While one of the ways to eliminate these obstacles is to add Li and O to the structure or to make a surface coating to stabilize the electrode/electrolyte interface, another solution is to control these transformations by doping the structure. Use of boron in doping becomes prominent as boron has high polarizing power due to its 3+ valence and small radius (0.098 nm.) and has strong and short bond length with oxygen leading to preventing oxygen release. Literature review reveals that two different strategies may be used to dop boron in the NMC chemistry. One is to doping boron in coprecipitation and the other is doping boron during calcination. The mechanism behind boron doping to the NMC structure during coprecipitation is that the (003) surface energy of the hydroxide is reduced compared to the {104} surfaces, thus providing the formation of primary particles oriented radially in this direction leading to the elongation of the crystallites in rod and needle shapes. Here in, the amount of boron doping is known to be crucial in the crystallization as B3+ cation may position in tetrahedral and octahedral sites (CN = 4 for 0.11 Å and CN = 6 for 0.27 Å) due to their small ionic radius. While the B3+ ion positioned in the octahedral sites will cause the cell to shrinkage in the c direction due to its small ionic radius compared to the TM ions but the B3+ ion positioned in the tetrahedral sites in the Li layer will cause expansion in the a and c direction. Moreover, the boron doping in calcination with lithium hydroxide reduces the lithium ions in the structure and prevents the development in the (003) direction, thus preventing the formation of the desired layered structure. The chemistry and amount of boron doping are quite effective in the performance of NMC 811. In NMC structures, by-products such as Li3BO3, LiBO2, Li2B4O7 are obtained as a result of the heat treatment of boron source with LiOH. It is observed that the formation of these by-products increases the cation mixing in the structure as the lithium consumption. Li-rich heat treatments are preferred in boron doping studies. In this study, a investigation is realized to investigate the effect of boron doping on NMC811 electrochemical performance. By adding boron in the coprecipitation and calcination steps of the cathode active material production process, the hypothesis put forth here is to examine the impact of the different characteristics of the B-doped NMC811 material on the electrochemical performance. No doping applied precursor named as 'NMC811OH' and boron-doped NMC811 named as 'NMC811OH1B' were successfully synthesized by co-precipitation method. NMC811OH and NMC811OH1B secondary particles with a size of approximately 10-15 μm and a spherical structure were produced. Galvanostatic tests reveal that NMC811 cathode active material without boron doping and calcined in air atmosphere (named as NMC811OH-air) delivers 160 mAh/g first charge capacity at C/10 and after 100 cycles at C/3 and C capacity retention are found to be 78% and 88% respectively, NMC811 cathode active material without boron doping and calcined in oxygen atmosphere so called NMC811OH-Ox delivers 203 mAh/g first charge capacity at C/10 and after 100 cycles at C/3 and C capacity retention are found to be 96.4% and 94.7% respectively. NMC811 cathode active material with boron doping (H3BO3) during co-precipitation and calcined in oxygen atmosphere named as NMC811OH1B-Ox delivers 188 mAh/g first charge capacity at C/10 and after 100 cycles at C/3 and C capacity retention are found to be 88% and 92% respectively and NMC811OH is mixed with LiOH and boron source (H3BO3) during calcination so called NMC811OH-Ox1B delivers 156 mAh/g first charge capacity at C/10 and after 100 cycles at C/3 and C capacity retention are found to be 88% and 89.67% respectively. I(003)/(104) ratio (inverse relationship with cation mixing) are found to be 1.22, 1.24, 1.22 and 1.17 for NMC811OH-air, named as NMC811OH-Ox, NMC811OH1B-Ox and NMC811OH-Ox1B. In all conditions calcined particles are sustained their spherical particle morphology and sizes being in the range of 10-15 μm. The galvanostatic performance shows that 1% H3BO3 doping during coprecipitation was insufficient to improve capacity retention compared to no doped NMC811OH. To further study the interaction of CAM with Li, potentiostatic cyclic voltammetry test is applied. For each electrode, peaks related to H1-M, M-H2 and H2-H3 transformations are noted, as expected. A right shift was detected in the H2-H3 peak in boron doped samples, but after 4 cycles, the loss in capacity peak intensity was found to be greater than the no doped sample in an oxygen environment and was associated with the oxygen loss in the structure and the capacity loss in cycle tests. The findings of this work highlights the importance of firstly Li/TM amount for boron source mixed with lithium hydroxide in calcination causing lithium deficiency caused by by-products or coating on the surface leading to cation mixing, poor charge-discharge capacity, capacity retention and high impedance. Boron doped via co-precipitation NMC811 sample have showed better electrochemical performance caused by better secondary particle morphology but 1% H3BO3 doping during coprecipitation is insufficient to improve capacity retention compared to undoped NMC811 sample. In the calcination processes carried out in oxygen and air, higher capacity was obtained due to the denser and more oriented primary particle structure with the presence of an oxygen-rich environment and better capacity retention due to the pore distribution in the internal structure and particle orientation.