First-principles Investigation of Lithium Intercalation and Transport in Manganese-oxides

The transportation and energy storage industries are currently undergoing a technology revolution involving electrification of every market segment. Top automakers such as GM, Ford, and Tesla are investing upwards of $30 billion each, building the material supply chain and battery manufacturing capabilities to accommodate the expected growth. As a result of this intense effort, the battery prices have reduced from $1160/kWh in 2010 to $150/kWh in 2020. Lithium (Li) intercalation chemistry has remained the gold standard since its discovery in 1972. Layered oxides such as lithium cobalt oxide, derivatives of lithium nickel aluminum oxides have increased the energy density to ~260 Wh/kg and offer up to 1000 full depth-of-discharge cycles. However, there are several remaining challenges: (i) the safety concerns associated with energy-dense chemistries such as nickel-manganese-cobalt oxide (e.g., LiNi0.8Mn0.1Co0.1O2) have proven to be challenging to solve. This issue necessitates complex battery cooling requirements in an EV application, (ii) batteries based on intercalation chemistry utilize less than 30% of their theoretical capacity: for example, lithium cobalt oxide/graphite in 18650 format has a theoretical volumetric energy density of 2950 WhL-1, but only 20% of it is utilized, resulting in an energy density of 570 WhL-1. The specific capacities of cathodes have been a prime limiting factor in improving the performance characteristics of modern Li-ion batteries. In addition, excessive reliance on Nickel and Cobalt has caused a supply chain bottleneck and instability in commodity pricing. So, there has been a coordinated effort from academia and industry to eliminate the expensive and limited-supply transition metals, which further require toxic chemicals for processing, from cathodes. In this context, there is a renaissance of cathode chemistries such as Mn-oxides that are inexpensive and non-toxic. Although positive electrode materials made of Mn (spinel, LiMn2O4) have been in usage since the 1980s, other polymorphs such as tunnel structured Mn-oxides have not found broader application in Li-ion intercalation chemistry. Understanding the key structure-property evolution in the context of intercalation processes and Li-ion transport in Mn-oxides with [2×2] tunnel structures form the basis of this dissertation work. [2×2] α-MnO2 material was chosen because of its varied electrochemical properties and its application as a 3V cathode with a potential specific capacity of ~ 504 Wh kg-1. These materials offer a larger annular space within their channels for ionic transport (in comparison to their [1x1] counterparts], and the surface facets are conducive for catalytic processes. At the same time, the α-MnO2 material system suffers from capacity fade due to structural degradation and loss of tunnel geometry. The underpinning physics behind such material degradation is not well understood. This dissertation has resulted in three different research manuscripts addressing several unanswered questions in regards to the electrochemical performance of α-MnO2. The research community is still investigating the exact role of cationic stabilization of tunnels and its role in structural degradation. To address this knowledge gap and offer theoretical insights, we studied the effect of stabilizing cations (K+) on the lithiation-induced evolution in structure and electronic as well as ion transport properties of the underlying material system. This study considered different K+-ion concentrations in α-KxMn8O16 and found that higher cationic content (2 K+/tunnel) impedes intercalation and transforms the structure into the layered configuration, leading to poor capacity retention. The discharge voltage predicted by Density Functional Theory (DFT) calculations is in agreement with the experimentally reported values in literature. Surface reconstruction and Mn-dissolution are a few of the key degradation mechanisms in Mn-rich electrodes. When this polymorph is synthesized and used in nanoscopic sizes, the high surface-to-volume ratio alters the interfacial diffusion dynamics, and the factors that lead to the dissolution of Mn at the surface under such size regimes and conditions are not well understood. So, this thesis directed substantive efforts towards understanding the nature of surface facets of these nanoscopic materials and offered important new insights. We found that 3D radial diffusion is a major limiting factor in realizing higher diffusivity and capacity retention. Finally, the role of cationic size and valence on Li-ion intercalation and capacity retention has been elucidated, and the findings from our work offer guidelines for the synthesis of [2×2] tunnel structures with different stabilizing cations. Overall, this body of work offers several theoretical insights into the structure-property relationship of α-MnO2 in the context of rechargeable batteries.