Alpesh K. Shukla

The effect of particle surface facets on the kinetic properties of LiMn1.5Ni0.5O4 cathode materials

Large LiMn1.5Ni0.5O4 single crystals in plate shape with (112) surface facets and octahedral shape with (111) surface facets were obtained by molten-salt synthesis. The presence of transition-metal ordering in both samples was independently confirmed by SAED, FTIR, NMR, and electrochemical studies, demonstrating the excellent capability of each technique in distinguishing the ordered and disordered phases. The apparent chemical diffusion minima during Li extraction and insertion were correlated with the occurrence of the first-order phase transition, implying that phase boundary movement limits Li transport in the spinel cathodes. Despite a more ordered structure, nearly ten times less Mn3+ content, and increased two-phase boundary movement during delithiation and relithiation, the octahedral crystals exhibited superior rate capability and a larger chemical diffusion coefficient, suggesting the kinetic preeminence of (111) surface facets over (112). The dominating effect of particle morphology and the importance of morphology design in achieving optimal performance of the LiMn1.5Ni0.5O4 spinel are clearly demonstrated for the first time.

Monodisperse Sn Nanocrystals as a Platform for the Study of Mechanical Damage during Electrochemical Reactions with Li

Monodisperse Sn spherical nanocrystals of 10.0 ± 0.2 nm were prepared in dispersible colloidal form. They were used as a model platform to study the impact of size on the accommodation of colossal volume changes during electrochemical lithiation using ex situ transmission electron microscopy (TEM). Significant mechanical damage was observed after full lithiation, indicating that even crystals at these very small dimensions are not sufficient to prevent particle pulverization that compromises electrode durability.

Unravelling structural ambiguities in lithium- and manganese-rich transition metal oxides

Although Li- and Mn-rich transition metal oxides have been extensively studied as high-capacity cathode materials for Li-ion batteries, the crystal structure of these materials in their pristine state is not yet fully understood. Here we apply complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li1.2(Ni0.13Mn0.54Co0.13)O2 crystals with two different morphologies as well as two commercially available materials with similar compositions, and unambiguously describe the structural make-up of these samples. Systematically observing the entire primary particles along multiple zone axes reveals that they are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets. More specifically, we show the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure, while the surface is composed of a Co- and/or Ni-rich spinel with antisite defects.

Improved kinetics and stabilities in Mg-substituted LiMnPO4

LiMgxMn1−xPO4 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) crystals were prepared hydrothermally. The presence of Mg2+ was found to improve the kinetics, utilization, and physical stability of the crystals during chemical and electrochemical delithiation, as well as the thermal stability of the delithiated phase. The best performance was found in the sample with 20% substitution. The positive effect of Mg2+ was attributed to the reduced volume mismatch between the lithiated and delithiated phases, and to more favorable particle morphologies. Mg2+ dilutes the concentration of Jahn–Teller active ion, Mn3+, and reduces local strains between the phases, and thereby increases the structural stability of the crystals. The result is a reduction in fracturing and decrepitation, which translates to improved electrochemical performance. Although the thermal stability improved with increasing Mg substitution, the heat evolved during reaction with electrolyte remains proportional to the Mn content and therefore to the theoretical capacity.

Asymmetric pathways in the electrochemical conversion reaction of NiO as battery electrode with high storage capacity

Electrochemical conversion reactions of transition metal compounds create opportunities for large energy storage capabilities exceeding modern Li-ion batteries. However, for practical electrodes to be envisaged, a detailed understanding of their mechanisms is needed, especially vis-à-vis the voltage hysteresis observed between reduction and oxidation. Here, we present such insight at scales from local atomic arrangements to whole electrodes. NiO was chosen as a simple model system. The most important finding is that the voltage hysteresis has its origin in the differing chemical pathways during reduction and oxidation. This asymmetry is enabled by the presence of small metallic clusters and, thus, is likely to apply to other transition metal oxide systems. The presence of nanoparticles also influences the electrochemical activity of the electrolyte and its degradation products and can create differences in transport properties within an electrode, resulting in localized reactions around converted domains that lead to compositional inhomogeneities at the microscale.

Study of Structure of Li- and Mn-rich Transition Metal Oxides Using 4D-STEM

The structure of Liand Mn-rich transition metal oxides (Li1+xM1-xO2, where M is usually a combination of transition metals such as Mn, Co and Ni, called LMRTMO henceforth) has been debated extensively for the past several years. It has been recently shown, by imaging entire primary particles at atomic resolution at multiple zone axes using high angle annular dark field (HAADF-) scanning transmission electron microscopy (STEM), that the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure [1] as shown in Figure 1 (a). Using HAADF-STEM, it was demonstrated that the particles did not contain two phases (trigonal and monoclinic) in the bulk as described earlier [2], since the size of the primary particles was small enough show that the monoclinic phase was the only phase present in entire particles. However, larger sized particles are often preferred for the cathode materials in order to obtain better volumetric energy density. For these applications, imaging entire particles at atomic resolution is very difficult and in some cases impossible owing to regions of higher thickness. In this paper we demonstrate the use of 4D-STEM [3] using large fields of view on a commercial cathode material to confirm that the bulk of the primary particles is made up of a single phase and consists of domains corresponding to three variants of monoclinic phase.

Effect of composition on the structure of lithium- and manganese-rich transition metal oxides

The choice of chemical composition of lithium- and manganese-rich transition metal oxides used as cathode materials in lithium-ion batteries can significantly impact their long-term viability as storage solutions for clean energy automotive applications. Their structure has been widely debated: conflicting conclusions drawn from individual studies often considering different compositions have made it challenging to reach a consensus and inform future research. Here, complementary electron microscopy techniques over a wide range of length scales reveal the effect of lithium-to-transition metal-ratio on the surface and bulk structure of these materials. We found that decreasing the lithium-to-transition metal-ratio resulted in a significant change in terms of order and atomic-level local composition in the bulk of these cathode materials. However, throughout the composition range studied, the materials consisted solely of a monoclinic phase, with lower lithium content materials showing more chemical ordering defects. In contrast, the spinel-structured surface present on specific crystallographic facets exhibited no noticeable structural change when varying the ratio of lithium to transition metal. The structural observations from this study warrant a reexamination of commonly assumed models linking poor electrochemical performance with bulk and surface structure.

A Multiple-Technique Approach for Resolving the Surface Structure of Lithium and Manganese Rich Transition Metal Oxides

High-capacity lithium transition-metal oxides (Li1+xM1-xO2, where M is usually a combination of transition metals such as Mn, Co and Ni) have been extensively studied recently due to their potential application in high-energy Li-ion batteries. Structural differences observed on the bulk and the surface, particularly the enrichment of Ni and/or Co and lower TM oxidation states on the surface, have been attributed to effects of cycling [1]. However, recent studies [2, 3] have shown that such transition metalrich surface layers are also observed in pristine materials. To ultimately achieve improved rate capability and stability of the batteries, it is necessary to fully understand the structure and composition of these surface layers. In this paper, we report the results from structural and spectroscopic analysis of these complex oxides using a variety of techniques that span a wide length-scale, including aberration corrected STEM, electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (XEDS).