Mechanical and Structural Degradation of LiNi_xMn_yCo_zO₂Cathode in Li-Ion Batteries: An Experimental Study

Abstract

LiNi x Mn y Co z O 2 (NMC) is the current choice of cathode for high-performance Li-ion batteries. The structural and mechanical stability of NMC plays a vital role in determining the electrochemical performance of batteries. However, the dynamic mechanical properties of NMC during Li reactions are widely unknown because of the microscopic heterogeneity of composite electrodes as well as the challenge of mechanical measurement for air-sensitive battery materials. We employ instrumented nanoindentation in an inert environment to measure the elastic modulus, hardness, and interfacial fracture strength of NMC of a hierarchical meatball structure as a function of the state of charge and cycle number. The mechanical properties significantly depend on the lithiation state and degrade as the electrochemical cycles proceed. The results are further compared with the properties of bulk NMC pellets. We perform first-principles theoretical modeling to understand the evolution of the elastic property of NMC on the basis of the electronic structure. This work presents the first time systematic mechanical measurement of NMC electrodes which characterizes damage accumulation in battery materials over cycles.

Mechanical, Electrical, and Ionic Behavior of Lithium‐Ion Battery Electrodes via Discrete Element Method Simulations

“…Specifically, Figure a shows the evolution of the coordination number during the first five cycles for all the electrodes, i.e., the average number of contacts per particle. Because the swelling and contraction of NMC particles was accompanied by changes in Young's modulus corresponding to the state of charge, the curves do not present a homogeneous form. In all cases, the maximum values of the coordination number were obtained at the beginning immediately after the calendering simulation was concluded.…”

“…Taking into consideration the changes in volume and mechanical properties of NMC particles, the evolution of the electrode structure was explored during lithium‐ion deintercalation and intercalation in subsequent simulations. This is of great interest due to the consequent impact on the transport and mechanical properties, which at the same time has an effect on the performance and life cycle of the battery cell . To this end and directly after the calendering simulations of these electrodes, the resulting structures were subjected to electrochemical cycling as fully explained in the Computational Section.…”

“…Thus, the lithiation‐dependent strain in each coordinate direction is equivalent. Likewise, the Young's modulus of NMC particles was adapted along the lithiation state as shown in the study by Xu et al The progressive reduction of the elastic modulus during delithiation was implemented starting with a value of 142.5 GPa for pristine NMC and ending with a value of 111.6 GPa for Li 0.5 [Ni 1/3 Mn 1/3 Co 1/3 ]O 2 . The swelling of particles was constrained by the neighboring particles that were expanding as well.…”

Fracture behavior in battery materials

“…Consequently, depth-dependent particle fracture is observed, which means a more sever crack occurs in the electrode near the separator more than near the current collector [92]. The experiment shows that the mechanical properties of a cathode degrade with a deep delithiation process and cycles [93], which is believed to be the consequence of state-of-charge heterogeneity and crack formation.…”

Operando Nanoindentation: A New Platform to Measure the Mechanical Properties of Electrodes during Electrochemical Reactions

“…In the future, it will help to unravel a variety of phenomena in energy materials involving the intimate interactions between mechanics and electrochemistry, such as stress-regulated ion diffusion and electron transfer, 66 concurrent processes of plasticity and reaction, 62,22 corrosive fracture, 67 and mechanical stability of electrodes in the long-term performance of batteries. 68 …”