Xiaochuan Lu

Electrochemical Energy Storage for Green Grid

The is a comprehensive review on the needs and potential storage technologies for electrical grid that is expected to integrate significant levels of renewables. This review offers details of the technologies, in terms of needs, status, challenges and future R&d directions.

Advanced intermediate-temperature Na–S battery

In this study, we reported an intermediate-temperature (∼150 °C) sodium–sulfur (Na–S) battery. With a relatively low operating temperature, this novel battery could reduce the cost and safety issues associated with the conventional high-temperature (300–350 °C) Na–S battery. A dense β′′-Al2O3 solid membrane and tetraglyme were utilized as the electrolyte separator and catholyte solvent in this battery. Solubility tests indicated that a cathode mixture of Na2S4 and S exhibited extremely high solubility in tetraglyme (e.g., >4.1 M for Na2S4 + 4 S). CV scans of Na2S4 in tetraglyme revealed two pairs of redox couples with peaks at around 2.22 and 1.75 V, corresponding to the redox reactions of polysulfide species. The discharge/charge profiles of the Na–S battery showed a slope region and a plateau, indicating multiple steps and cell reactions. In situ Raman measurements during battery operation suggested that polysulfide species were formed in the sequence of Na2S5 + S → Na2S5 + Na2S4 → Na2S4 + Na2S2 during discharge and in a reverse order during charge. This battery showed dramatic improvement in rate capacity and cycling stability over room-temperature Na–S batteries, which makes it more attractive for renewable energy integration and other grid related applications.

Liquid-metal electrode to enable ultra-low temperature sodium–beta alumina batteries for renewable energy storage

Commercial sodium-sulphur or sodium-metal halide batteries typically need an operating temperature of 300-350 °C, and one of the reasons is poor wettability of liquid sodium on the surface of beta alumina. Here we report an alloying strategy that can markedly improve the wetting, which allows the batteries to be operated at much lower temperatures. Our combined experimental and computational studies suggest that addition of caesium to sodium can markedly enhance the wettability. Single cells with Na-Cs alloy anodes exhibit great improvement in cycling life over those with pure sodium anodes at 175 and 150 °C. The cells show good performance even at as low as 95 °C. These results demonstrate that sodium-beta alumina batteries can be operated at much lower temperatures with successfully solving the wetting issue. This work also suggests a strategy to use liquid metals in advanced batteries that can avoid the intrinsic safety issues associated with dendrite formation.

A Low Cost, High Energy Density, and Long Cycle Life Potassium–Sulfur Battery for Grid‐Scale Energy Storage

A potassium-sulfur battery using K(+) -conducting beta-alumina as the electrolyte to separate a molten potassium metal anode and a sulfur cathode is presented. The results indicate that the battery can operate at as low as 150 °C with excellent performance. This study demonstrates a new type of high-performance metal-sulfur battery that is ideal for grid-scale energy-storage applications.

Cell degradation of a Na–NiCl2 (ZEBRA) battery

In this work, the parameters influencing the degradation of a Na–NiCl2 (ZEBRA) battery were investigated. Planar Na–NiCl2 cells using the β′′-alumina solid electrolyte (BASE) were tested with different C-rates, Ni/NaCl ratios, and capacity windows, in order to identify the key parameters for the degradation of the Na–NiCl2 battery. The morphology of NaCl and Ni particles was extensively investigated after 60 cycles under various test conditions using a scanning electron microscope. A strong correlation between the particle size (NaCl and Ni) and battery degradation was observed in this work. Even though the growth of both Ni and NaCl can influence the cell degradation, our results indicate that the growth of NaCl is a dominant factor in cell degradation. The use of excess Ni seems to play a role in tolerating the negative effects of particle growth on degradation since the available active surface area of Ni particles can still be sufficient even after particle growth. For NaCl, a large cycling window was the most significant factor, of which effects were amplified with decrease in the Ni/NaCl ratio.

Advanced intermediate temperature sodium–nickel chloride batteries with ultra-high energy density

Sodium-metal halide batteries have been considered as one of the more attractive technologies for stationary electrical energy storage, however, they are not used for broader applications despite their relatively well-known redox system. One of the roadblocks hindering market penetration is the high-operating temperature. Here we demonstrate that planar sodium–nickel chloride batteries can be operated at an intermediate temperature of 190 °C with ultra-high energy density. A specific energy density of 350 Wh kg−1, higher than that of conventional tubular sodium–nickel chloride batteries (280 °C), is obtained for planar sodium–nickel chloride batteries operated at 190 °C over a long-term cell test (1,000 cycles), and it attributed to the slower particle growth of the cathode materials at the lower operating temperature. Results reported here demonstrate that planar sodium–nickel chloride batteries operated at an intermediate temperature could greatly benefit this traditional energy storage technology by improving battery energy density, cycle life and reducing material costs.

A novel low-cost sodium–zinc chloride battery

The sodium–metal halide (ZEBRA) batteries have been considered as one of the most attractive energy storage systems for stationary and transportation applications. Even though the battery technologies have been widely investigated for a few decades, there is still a need to further improve the battery performance, cost and safety for practical applications. In the present work, a novel low-cost Na–ZnCl2 battery with a planar β′′-Al2O3 solid electrolyte (BASE) was proposed, and its electrochemical reactions and battery performance were investigated. Compared to Na–NiCl2 chemistry, the ZnCl2-based chemistry was more complicated, in which multiple electrochemical reactions including liquid-phase formation occurred at temperatures above 253 °C. During the first stage of charge, NaCl reacted with Zn to form Na in the anode and Na2ZnCl4 in the cathode. Once all the NaCl was consumed, further charge with the reaction between Na2ZnCl4 and Zn led to the formation of a NaCl–ZnCl2 liquid phase. During the end of charge, the liquid phase reacted with Zn to produce solid ZnCl2. To identify the effect of liquid-phase formation on electrochemical performance, button cells were assembled and tested at 280 and 240 °C. At 280 °C, with the liquid phase formed during cycling, cells revealed quite stable cyclability. On the other hand, more rapid increase in polarization was observed at 240 °C where only solid-state electrochemical reactions occurred. SEM analysis indicated that the stable performance at 280 °C was due to the suppressed growth of Zn and NaCl particles, which were generated from the liquid phase during the discharge of each cycle.

Chapter 1 – Electrochemical cells for medium- and large-scale energy storage: fundamentals

This chapter provides a comprehensive overview of the general fundamentals of electrochemical cells. During operation, the battery processes involve electron transfer at the electrode-electrolyte interface and are closely associated with the electrical double layer that affects the kinetics of electrode reactions. Although the theoretical cell potential of a battery is governed by the Nernst equation, the overpotential always exists in the actual operation of practical batteries. The three major origins of the cell overpotential and the governing laws are introduced individually: ohmic loss, electron transfer, and mass transfer. Other important battery parameters, such as capacity, efficiency, energy density, cycle life, and safety, are briefly discussed. Emphasis is given to the electrochemical fundamentals of three main types of batteries that currently undergo extensive research efforts: lithium-ion battery, redox flow battery, and sodium battery. The working principles, cell architectures, typical electroactive materials, battery reactions, and capacity fading mechanisms of these batteries are introduced in details.

Decorating β′′-alumina solid-state electrolytes with micron Pb spherical particles for improving Na wettability at lower temperatures

Overcoming poor physical contact is one of the most critical hurdles for batteries using solid-state electrolytes. In particular, overpotential from the liquid–solid interface between molten sodium and a β′′-alumina solid-state electrolyte (BASE) in a sodium–metal halide (Na–MH) battery could be enormous at lower operating temperatures (<200 °C) due to intrinsically poor Na wetting on the BASE surface. In this work, we describe how surface modification with lead acetate trihydrate (LAT) at different temperatures affects Na wetting on BASEs. LAT treatment conducted at a temperature of 400 °C (under a nitrogen gas atmosphere) shows significantly better Na wettability and battery performance than treatments at lower temperatures. The formation of a unique morphology—micron-sized Pb spherical particles—is observed on the surface of the BASE LAT treated at 400 °C. We also observed evolution of the Na wetting configuration from a Cassie drop, to a Wenzel drop, and finally to a sunny-side-up drop, which is clearly different from the Young–Dupre relation, with increasing the contact-angle measurement temperature. We conclude that formation of a thin Na penetrating film (sunny-side-up shape) on Pb-decorated BASEs is crucial for achieving good battery performance at lower operating temperatures. The new observations and fundamental understanding of Na wetting reported here will provide excellent guidance for improving cell performance in general and will further promote development of practical Na–MH battery technologies for large-scale energy storage applications.