Anirudh Ramanujapuram

Lithium‐ion batteries with aqueous electrolytes can be excellent candidates for battery applications at low temperatures. In contrast to a common misconception, aqueous lithium ion batteries can operate at several tens of degrees below the freezing point of water when high concentration electrolyte solutions are utilized. Furthermore, it is reported here that the performance of intercalation cathodes in aqueous electrolytes is quite remarkable and superior to that in common organic electrolytes at very low temperatures down to about −40 °C. Here in the performance of water‐based electrolyte solutions‐based on three low‐cost inorganic salts (LiNO₃, Li₂SO₄, and LiCl) and that of the corresponding aqueous battery systems is studied in order to understand the rate‐limiting step at sub‐zero temperatures. It is found that the charge transfer resistance is the largest impedance contributor at low temperatures. However, layered cathodes in aqueous electrolytes do not exhibit a significant increase in the charge‐transfer resistance, or a reduction in the accessible capacity during charging until the temperature is close to the solution freezing point. This is in sharp contrast to their behavior in organic electrolytes that do not support any performance below −20 °C. This different behavior explains the dramatically superior performance of lithium ion battery cathodes in water‐based electrolytes at lower temperatures.

With the most recent development of ultrahigh capacity anodes, such as Li‐ or Si‐based anodes, metal fluorides hold promise as complementary high‐capacity conversion cathode materials for next‐generation energy storage devices. Despite their higher theoretical energy density compared to cells with sulfur cathodes, these materials have received dramatically less attention and little is understood about the origins of their electrochemical behavior. Here, the successful methodology to produce highly uniform size‐controlled mixed metal difluoride nanocomposites is reported. It is discovered that such materials undergo reduction in a single step with a reduction potential intermediate to those for the corresponding single‐metal difluorides and that a solid solution is reformed upon charging, which is advantageous for practical applications. For the first time the progressive formation of metal trifluorides upon repeated cycling of difluorides is reported. Systematic electrochemical measurements in combination with postmortem analyses lead to the conclusion that the cathode stability strongly depends on the ability to prevent formation and growth of a resistive cathode solid electrolyte interphase, which, in turn, strongly depends on the metal composition. This methodology and new findings will help to elucidate a path to developing metal fluoride–based commercial Li‐ion batteries and provide guidelines for material selection.

Li‐halide hydroxides (Li₂OHX) and Li‐oxyhalides (Li₃OX) have emerged as new classes of low‐cost, lightweight solid state electrolytes (SSE) showing promising Li‐ion conductivities. The similarity in the lattice parameters between them, careless synthesis, and insufficient rigor in characterization often lead to erroneous interpretations of their compositions. Finally, moisture remaining in the synthesis or cell assembling environment and variability in the equivalent circuit models additionally contribute to significant errors in their properties. Thus, there remains a controversy about the real values of Li‐ion conductivities in such SSEs. Here an ultra‐fast synthesis and comprehensive material characterization is utilized to report on the ionic conductivities of contaminant‐free Li2+xOH_1−xCl (x=0‐0.7), and Li₂OHBr not exceeding 10^‐4 S cm^‐1 at 110 °C. Using powerful combination of experimental and numerical approaches, it is demonstrated that the presence of H in these SSEs yields significantly higher Li⁺ ‐ionic conductivity. Born‐Oppenheimer molecular dynamics simulations show excellent agreement with experimental results and reveal an unexpected mechanism for faster Li⁺ transport. It involves rotation of a short OH‐group in SSEs, which opens lower‐energy pathways for the formation of Frenkel defects and highly‐correlated Li⁺ jumps. These findings will reduce the existing confusions and show new avenues for tuning SSE compositions for further improved Li‐ion conductivities.

The development of an aqueous lithium ion battery (ALIB) has the potential to greatly improve the safety and lower the cost of lithium ion battery storage systems, while safeguarding the environment. The problem of thermal runaway and ignition of a flammable electrolyte is absent. Aqueous electrolytes exhibit higher ionic conductivities than their organic counterparts [1], allowing for the use thicker electrodes without suffering mass transport limitations. Combined, this promises to allow for a significant reduction in the amount of inactive components required to assemble a battery cell compared to a typical lithium ion battery cell with an organic electrolyte, helping to improve the final weight, volume and cost of an ALIB storage system [2]. Furthermore, the use of dry rooms and expensive and toxic organic electrolytes is avoided. However, prior to commercialization of ALIBs, several challenges need to be overcome. There have been limited studies into the unique side reactions that occur between active materials and aqueous electrolyte, along with methods for reducing the impact of these side reactions on the battery performance [3]. In addition, creative solutions must be found to improve the voltage of the battery in view of the more limited potential window over which aqueous electrolytes are stable [4]. Here, we share the results of our systematic studies on the interactions of lithium iron phosphate (LFP) with aqueous electrolytes of varying composition. Increasing the lithium salt concentration of the electrolyte was found to significantly increase the cycle stability. Post-mortem analyses using microscopy (SEM, TEM), spectroscopy (XRD, EDS, FTIR, XPS), and depth profiling (TOF-SIMS), as well as electrochemical characterization using EIS and charge-discharge, help to elucidate the origins of the capacity fade and reasons for improved cycling stability with higher salt concentrations [2]. We propose that side reactions between water molecules and LFP lead to electrochemical separation of individual particles in the electrode, and that increasing the electrolyte molarity effectively reduces the concentration of water molecules available for these side reactions. We believe that lessons learned while studying the behavior of LFP in aqueous solutions will likely guide a path toward better understanding and optimizing the behavior of other intercalation materials in ALIBs, as we have already observed similar impacts of the aqueous electrolyte composition on the stability of lithium cobalt oxide (LCO) and lithium nickel manganese cobalt oxide (NMC) [5]. References [1] J.-Y. Luo, W.-J. Cui, P. He, Y.-Y. Xia, Nat. Chem. 2010, 2, 760 [2] D. Gordon, Y. Wu, A. Ramanujapuram, J. Benson, J.T. Lee, A. Magasinski, N. Nitta, C. Huang and G. Yushin, Adv. Energy Mater. 2015, 2, 1501805 [3] Y. Wang, J. Yi, and Y. Xia, Adv. Energy Mater. 2012, 2, 830 [4] C. Wessells, R. Ruffo, R. A. Huggins, Y. Cui, Electrochem. Solid-State Lett. 2010, 13, A59 [5] A. Ramanujapuram, D. Gordon, A. Magasinski, B. Ward, N. Nitta, C. Huang, G. Yushin, Energy and Environmental Science 2016 [in press]

Lithium cobalt oxide (LCO) has long been proven to be an excellent material for cathodes in organic electrolytes [1]. It has shown high volumetric capacity and good stability in non-aqueous environments of commercial lithium-ion (Li-ion) batteries. Unfortunately, the flammability of organic electrolytes in combination with a special propensity for batteries constructed with LCO to experience thermal runaway creates safety concerns [2]. Here we discuss the exceptional electrochemical performance characteristics and stability of LCO in aqueous electrolytes. Aqueous electrolytes offer greatly improved safety and lower cost. In addition, they exhibit higher ionic motilities for the Li⁺ions and thus can be potentially used for faster charging batteries [3] or batteries with thicker electrodes. While LCO has been demonstrated to cycle for 20-100 cycles in aqueous environments [4, 5], the causes of its degradation have not been investigated in detail. In our previous work we have demonstrated promising characteristics of lithium iron phosphate (LFP) cathodes in aqueous electrolytes [6]. Here we further expanded our studies and demonstrated LCO cathodes with remarkably stable performance showing only 13 % fading after over 1500 cycles. Post mortem analysis of the electrodes was conducted to understand the effect of cycling and the causes of degradation. A powerful combination of X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) have been utilized. Our systematic studies and post-mortem analyses of LCO cells reveal that the primary mechanism of LCO degradation in aqueous electrolytes is the formation of a resistive layer of cobalt (II) oxide on the particles’ surfaces. Electrolyte composition was found to have a dramatic impact on the electrochemical performance and stability of LCO in aqueous environments. These results were recently accepted for publication in the Energy and Environmental Science Journal [7]. This talk will provide an overview of our findings and provide guidance for the future designs of aqueous electrochemical cells with stable cathodes. References: 1. J. Zhang, Y.J. Xiang, Y. Yu, S. Xie, G.S. Jiang, C.H. Chen *, Journal of Power Sources 132 (2004) 187–194 2. J.R. Dahn, E.W. Fuller, M. Obrovac, U. von Sacken, Solid State Ionics 69 (1994,) 265-270 3. Y. Wang, J. Yi , and Y. Xia*, Advanced Energy Materials 2012, 2, 830–840 4. G. Wang, L. Fu, N. Zhao, L. Yang, Y. Wu,* and H. Wu, Angew. Chem. Int. Ed. 2007, 46, 295–297 R. Ruffo, C. Wessells, R. A. Huggins, Yi Cui*, Electrochemistry Communications 11 (2009) 247–249 6. D. Gordon, Y. Wu, A. Ramanujapuram, J. Benson, J.T. Lee, A. Magasinski, N. Nitta, J. Huang and G. Yushin, Advanced Energy Materials (2015) 1501805 7. A. Ramanujapuram, D. Gordon, A. Magasinski, B. Ward, N. Nitta, C. Huang, G. Yushin, Energy and Environmental Science (2016) [in press]

We report herein the exceptional cycle stability of lithium cobalt oxide (LCO) in aqueous electrolytes of high lithium salt concentrations.

Aqueous lithium ion batteries (ALIBs) exhibit great potential to reduce the cost and improve the safety of rechargeable energy storage technologies. Lithium iron phosphate (LFP) cathodes have become a material of choice for many conventional, high power LIBs. However, experimental studies on LFP in aqueous lithium (Li) ion electrolytes are limited. Here, results of systematic studies are shown where it is demonstrated that the Li salt concentration of the aqueous electrolyte can significantly improve discharge capacity retention while minimally impacting rate capability, for electrodes made with a typical commercial sub‐micron sized LFP powder. Based on the postmortem analysis and the results of electrochemical characterization it is proposed that undesirable side reactions of aqueous electrolytes with LFP induce electrochemical separation of individual particles within the electrode, leading to the observed capacity fading. Increasing the salt concentration in aqueous solutions effectively reduces the concentration of water molecules in the electrolyte, which are mostly responsible for these undesirable side reactions. Similar trends observed with other cathode materials suggest that the use of concentrated aqueous electrolyte solutions offers an effective route to improve stability of aqueous Li ion batteries.

An ice templating coupled with hard templating and physical activation approach is reported for the synthesis of hierarchically porous carbon monoliths with tunable porosities across all three length scales (macro- meso- and micro), with ultrahigh specific pore volumes ∼11.4 cm³ g⁻¹. The materials function well as amine impregnated supports for CO₂ capture and as supercapacitor electrodes.