Enhancement of energy efficiency of high-nickel layered oxide cathodes via solution-based primary-particle-level coating

Abstract

Recent advances in lithium-ion batteries (LIBs) technology have achieved significant performance improvement. Nevertheless, energy density improvement and cost reduction are still necessary to obtain widespread adoption of electric vehicles. Accordingly, low-cobalt active materials, such as nickel-rich lithium nickel manganese cobalt oxide (Ni-rich NMC, $LiNi_xMn_{1-x-y}Co_yO_2$, $x \geq 0.8$, $y \leq 0.1$) have been extensively studied as a cathode material. Moreover, use of higher upper cutoff voltage is considered effective to further enhance the energy density of NMC. However, the increases in Ni content and cutoff voltage lead to poor cyclability. High-Ni content and deep charging of NMC accelerate side reactions on the electrode surface such as rock-salt-like phase formation and transition metal dissolution. In addition, the deep charging is accompanied by severe volume change of NMC particles, that is signified with increasing Ni content. The volume changes during delithiation and lithiation cycles generate strain in the material particles, resulting in formation of intergranular cracks due to the anisotropic expansion and shrinkage of each primary particles. This study demonstrates that formation of dislocation is greatly affected by the diffusion-induced Li concentration gradients in the NMC primary particles. In that respect, reducing primary particle sizes of NMC is an effective way to suppress formation of dislocation. However, smaller primary particles facilitate side reactions on their surfaces. In this study, to improve cycle performance of high-capacity Ni-rich NMC, we present a novel method to reduce primary particle sizes with efficiently suppressing the side reactions on the surface. 1) Enhancing energy efficiency of $LiNiO_2$ cathode for Li-ion battery via suppressing surface-side-reaction using primary-particle-level $Li_2SO_4$ coating Primary-particle-level $Li_2SO_4$ coated $LiNiO_2$ (SLNO) was successfully synthesized using low cost and simple solution-based coating. Here, we focused on the formation mechanism of $Li_2SO_4$ coating layer and its effects on the formation of reconstruction layer and transition metal dissolution. Compared to conventional coating methods, which conducted after calcination process, our coating process was conducted before calcination process. After calcination at high temperature, the particles is condensed, and hence coating material could not permeated into the secondary particle inside. Contrarily, the $Ni(OH)_2$ precursor is loosely packed and has sufficient pores inside, which allows the primary-particle-level coating. Even after calcination process, it is confirmed that the coating layer exist along the primary particle selectively. The SLNO electrode shows much better energy efficiency of 92.5%, whereas the uncoated $LiNiO_2$ (LNO) shows 86.4%. Through the ex-situ HRTEM analysis, we confirmed that much thinner reconstruction layer is formed in SLNO than LNO after 100 cycles. It suggests that the $Li_2SO_4$ coating layer successfully protect the cathode particle from HF attack and/or suppress $O_2$ evolution. 2) Strain-induced dislocation formation in the primary particle under deep charging and its effects on the electrochemical performance of Ni-rich cathode

suppressing dislocation formation via reducing primary particle size In this study, we propose the novel way to suppress the formation of crystal dislocation, which greatly suppresses the Li diffusion in the materials, by reducing primary particle size of Ni-rich and Co-free cathode via facile and low-cost $NiSO_4$ soaking method. Since the Li diffusion path in the smaller primary particle size is shorter than large one, the Li concentration gradient in the primary particle can be mitigated by reducing primary particle size. It means that the particle undergoes homogeneous phase transition, resulting in less diffusion induced strain formation. However, reducing particle size always leads to the increase in surface area, leading to a more side reactions such as rock-salt-like phase formation and transition-metal dissolution. Our method not only reduce the primary particle size but also suppress the side reaction by leaving primary-particle-level $Li_2SO_4$ thin layer. Indeed, we confirmed that the less strain is induced in the particles after cycling, and accordingly, less dislocation is formed. The sample with smaller primary particle size shows less reduction in Li diffusivity upon cycling, and hence it exhibits much less drop of electrode potential and better cycling stability than untreated counterpart. Furthermore, it is confirmed that the $Li_2SO_4$ coating layer suppress the surface side reaction effectively. 3) Effect of cathode-side $Li_2SO_4$ coating on the structural stability of nanostructured anode material in the full-cell The transition metal dissolution at the cathode surface is accompanied by the pH drop of electrolyte because the reaction forms $H_2O$, which can generate further HF by reacting with Li salts. Nowadays, because of the limited capacity of conventional graphite anode, high-capacity anode materials such as alloying-reaction-based, and conversion-reaction-based materials have been widely studied. The high-capacity anode materials generally undergo severe volume change upon lithiaion/delithiation, leading to the crack formation and pulverization. In order to overcome the volume change issue, various nanostructured materials were adopted. However, in the perspective of full-cell, the nanostructured anode material is highly susceptible to the acid leaching due to their high surface area. In order to confirm the effect of electrolyte pH drop on the nanostructured anode materials, we prepared nanostructured Sb2S3 nanorods, and performed full-cell test with LNMO and SLNMO. The more loss of $Sb_2S_3$ nanorods was clearly shown after full-cell test with LNMO than SLNMO. Moreover, through the simulation test, it is confirmed that the $Li_2SO_4$ coating significantly suppresses the pH drop of electrolyte.