Reducing cost and increasing energy density are two barriers for widespread application of lithium-ion batteries in electric vehicles. Although the cost of electric vehicle batteries has been reduced by $70% from 2008 to 2015, the current battery pack cost ($268/kWh in 2015) is still >2 times what the USABC targets ($125/kWh). Even though many advancements in cell chemistry have been realized since the lithium-ion battery was first commercialized in 1991, few major breakthroughs have occurred in the past decade. Therefore, future cost reduction will rely on cell manufacturing and broader market acceptance. This article discusses three major aspects for cost reduction: (1) quality control to minimize scrap rate in cell manufacturing; (2) novel electrode processing and engineering to reduce processing cost and increase energy density and throughputs; and (3) material development and optimization for lithium-ion batteries with high-energy density. Insights on increasing energy and power densities of lithium-ion batteries are also addressed.

To correlate the carbon properties, e.g. surface area and porous structure, with the electrochemical behaviors of carbon/sulfur (C/S) composite cathodes for lithium-sulfur (Li-S) batteries, four different carbon frameworks including Ketjen Black (KB, high surface area and porous), Graphene (high surface area and nonporous), Acetylene Black (AB, low surface area and nonporous) and Hollow Carbon Nano Sphere (HCNS, low surface area and porous) are employed to immobilize sulfur (80 wt%). It has been revealed that high surface area of carbon improves the utilization rate of active sulfur and decreases the real current density during the electrochemical reactions. Accordingly, increased reversible capacities and reduced polarization are observed for high surface area carbon hosts such as KB/S and graphene/S composites. The porous structure of KB or HCNS matrix promotes the long-term cycling stability of C/S composites but only at relatively low rate (0.2 C). Once the current density increases, the pore effect completely disappears and all Li-S batteries show similar trend of capacity degradation regardless of the different carbon hosts used in the cathodes. The reason has been assigned to the formation of reduced amount of irreversible Li2S on the cathode as well as shortened time for polysulfides to transport toward lithium anode at elevated current densities. This work provides valuable information for predictive selection on carbon materials to construct C/S composite for practical applications from the electrochemical point of view.

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Silicon-graphite composites are under development for the next generation of high-capacity lithium-ion anodes, and vibrational spectroscopy is a powerful tool to identify the different mechanisms that contribute to performance loss. With alloy anodes, the underlying causes of cell failure are significantly different in half-cells with lithium metal counter electrodes compared to full cells with standard cathodes. However, most studies which take advantage of vibrational spectroscopy have only examined half-cells. In this work, a combination of FTIR and Raman spectroscopy describes several factors that lead to degradation in full pouch cells with LiNiMnCoO (NMC532) cathodes. The spectroscopic signatures evolve after longer term cycling compared to the initial formation cycles. Several side-reactions that consume lithium ions have clear FTIR signatures, and comparison to a library of reference compounds facilitates identification. Raman microspectroscopy combined with mapping shows that the composite anodes are not homogeneous but segregate into graphite-rich and silicon-rich phases. Lithiation does not proceed uniformly either. A basis analysis of Raman maps identifies electrochemically inactive regions of the anodes. The spectroscopic results presented here emphasize the importance of improving electrode processing and SEI stability to enable practical composite anodes with high silicon loadings.

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In this work, we explore how the chemical reactivity toward an aprotic battery electrolyte changes as a function of lithium salt and silicon surface termination chemistry. The reactions are highly correlated, where one decomposition reaction leads to a subsequent decomposition reaction. The data show that the presence of silicon hydrides (SiH x ) promotes the formation of CO gas, while surface oxides SiO x drive the formation of CO2. The extent and rate of oxidation depend on the surface basicity of the SiO2 surface species. The most acidic surfaces seem to hinder CO2 generation but not the decomposition of the salt. Indeed, the presence of F-containing salts (LiPF6 and LiTFSI) promotes the reactions between carbonate electrolyte and silicon surfaces. Surfaces with high Li content seem to be the most passivating to gassing reactions, pointing to a pathway to stabilize the interfaces during cell formation and assembly.

Si has the possibility to greatly increase the energy density of Li-ion battery anodes, though it is not without its problems. One issue often overlooked is the decomposition of Si during large scale slurry formulation and battery fabrication. Here, we investigate the mechanism of H 2 production to understand the role of different slurry components and their impact on the Si oxidation and surface chemistry. Mass spectrometry and in situ pressure monitoring identifies that carbon black plays a major role in promoting the oxidation of Si and generation of H 2 . Si oxidation also occurs through atmospheric O 2 consumption. Both pathways, along with solvent choice, impact the surface silanol chemistry, as analyzed by 1 H− 29 Si cross-polarization magic angle spinning nuclear magnetic resonance (MAS NMR) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR). An understanding of the oxidation of Si, during slurry processing, provides a pathway toward improving the manufacturing of Si based anodes by maximizing its capacity and minimizing safety hazards.

Understanding the aprotic solution structures at the immediate vicinity of solid/liquid interface (SLI) is critically important for next generation lithium ion battery development. Yet, it is still challenging to investigate the carbonate chemical profiles close to the diffuse layer (about 10 nm) of the electrical double layer at SLI due to the lack of a ultrahigh surface sensitive tool. In this work, we demonstrate the structures of commonly used carbonate solvents (ethylene carbonate (EC) and diethyl carbonate (DEC)) and a carbonate additive (fluoroethylene carbonate (FEC)) in a commercial Li-ion battery electrolyte can be determined at ∼17 nm above the electrode surface. This is only enabled by a nanogap surface-enhanced Raman spectroscopy (SERS) technique based on a monolayer gold nanoparticle (Au NP) ensemble. The SERS enhancement factor (EF) of those carbonates was found to depend on the molecular polarizability, with the maximum EF at ∼10 5 found for EC and FEC. Despite their alike chemical structures, this monolayer Au NP SERS substrate is fully capable of discrimiating the different Raman finger prints of EC and FEC. Compared to EC, several vibration modes in FEC, such as C-C skeletal deformation, ring breathing band and C=O stretching band, shift to higher frequencies because of the displacement of a hydrogen atom by a much heavier fluorine atom in a methylene bridge. This counterintuitive observation against the commonly used "ball and spring" model in vibrational spectroscopy is mostly due to the increased bond strength in the FEC ring versus that of EC. A second order empirical polynomial best describes the correlation between the SERS band integration of EC or DEC molar concentration. Our findings open up new opportunities for in-depth understanding of the electrolyte molecular vibrational behaviors at direct solid/liquid interface and developing advanced electrolytes for next generation lithium-ion batteries.