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Zinc-Bromine Rechargeable Batteries - Nano-Micro Letters (2023)

Glossary, safety and general information

• GF = Graphite Felt
• Anolyte : gives up $e⁻$
• Catholyte : accepts $e⁻$
• Membrane : lets $p⁺$ through
• An aqueous solution of dissolved ${\text{ZnBr}}_2$ 1-4 M) is used as the electrolyte in most ZBRBs
• Bromine smells like bleach, and $B{r}_3⁻$ or polyhalide ions are highly oxidising
• Bromine shuttle effect : diffusion of polybromide ions (resulting from ${\text{Br}}_2$ binding with $\text{Br}⁻$) from cathode to anode, leading to battery degradation (self discharge, capacity fade)
• SoC = State of Charge (of the battery) : higher SoC increases overpotentials

Zinc bromine rechargeable batteries (ZBRBs) are hybrid batteries : some of the energy is stored at the negative electrode (anode) via metallic zinc plated during the charging phase, while the remaining energy is stored in a liquid phase at the catholyte. They come in two configurations : static (cheaper, no pumping and lesser maintenance) and flow (higher efficiency). This study concentrates on flow ZBRBs.

Supporting Reactions

Anode side

• ${\text{Zn}}^{2+}$ + 2e⁻ → ${\text{Zn}}^0$ (Charging)
• ${\text{Zn}}^0$ → ${\text{Zn}}^{2+}$ + 2e⁻ (Discharging)
• E°= −0.763 vs SHE at 25°C

Cathode side

• 2$\text{Br}⁻$ → ${\text{Br}}_2$ + 2e⁻ (Charging)
• ${\text{Br}}_2$ + 2e⁻ → 2$\text{Br}⁻$ (Discharging)
• E°=+1.065 V vs SHE at 25°C

Limitations of the technology

1. Zinc dendrite growth resulting from repeated electroplating and stripping of zinc that can pierce the membrane and eventually forms a conductive bridge between the electrodes (shorting)
2. hydrogen gas generation as the electrochemical potential of charge/discharge process of the system which is higher than that required for water hydrolysis which competes with the reduction reaction of Zn2+ ions and decreases the overall efficiency of the ZBRBs,
3. corrosive elemental bromine liquid, Br 2 (l), production at the positive electrode during charge, which can be diffused through the membrane to the zinc half-cell reacting with the Zn plated at the negative electrode (crossover), causing self-discharge and/or degradation
4. the low miscibility (~ 2.8 vol%) and stratification behaviour of Br2(l) in aqueous solutions that can lead to non-uniform concentration distribution

Static ZBRBs

• Held back by high self-discharge rate and low energy density (microbatteries suffer)
• glass fibre separators work well (instead of porous carbon)
• Gelion Endure™ company makes gel ZBRBs, but sensitive to temperatures over 50 °C
• Carbon foam electrode : highly porous flexible carbon foam

Redox Flow Battery (RFB)

• rapid response times, measured in milliseconds => these systems are well suited for levelling intermittent renewable power output
• VFRB (vanadium) are well-developed and commercialised
• ZBFB has substantial advantages over other flow batteries, such as high energy density, high cell voltage and the low cost of the materials

• Negative electrode (zinc), positive electrode (bromine), separated by a membrane to prevent cross-contamination.
• Two tanks of aqueous electrolyte solutions (anolyte and catholyte) contain electrochemically active species, including zinc (Zn2+) and bromide (Br-)
• Elemental bromine exists in equilibrium with bromide ions forming polybromide ions, $B{r}_n⁻$, where n = 3, 5 and 7
• two pumps to circulate the electrolyte solutions over both electrode surfaces, controlling generated heat, feeding and homogenising reactants, removing bromine complexes from the stack and ensuring uniform zinc deposits
• Single pump systems exist

Kinetics

• Behaviour and kinetics of zinc cations are strongly affected by other supporting electrolytes in aqueous solutions containing bromide

Dendritic growth (page 8)

• It is important to strip the zinc in ZBRBs for extended periods to ensure a smooth electrode surface for next zinc deposition.
• Residual zinc left on the anode after discharge results in the loss of 3–5% of the amp-hour capacity
• However, the remaining zinc could potentially be used as a useful energy source if additional zinc is plated over it in the subsequent cycles.
  • @ What? Nucleation sites? Of course it can be stripped after, that doesn't make it additional energy
• Effect of operating temperature on zinc deposits : zinc deposits (grey in appearance) turned black at temperatures higher than 40 °C
• Smooth and bright zinc deposits were obtained when increasing the electrolyte’s zinc concentration
• In the initial stage, zinc deposition begins with nucleation and continues with growth, meaning the formation of dendrites is a cumulative result of battery cycling, not a single cycle.
• Zinc ions will deposit on a zinc nucleus rather than nucleating at a new site since zinc nucleation has a higher overpotential than zinc growth
• Formation of zinc dendrites can damage the membrane

• Dendrites form on the membrane side dominantly

Dendrite Mitigation Strategies

• Thermal treatment of GF => higher defect concentration => better zinc diffusion
• Aqueous electrolyte additives => reduced surface reactions
• Another proposed strategy involves creating an artificial interfacial layer between the zinc and the electrolyte that performs the same function as an SEI
• Metal organic framework (MOF) material with unique channel structure can be used as coating layers to modify Zn anode through tuning specific chemical composition and porous structure. The ZIF-7 channel can prevent large solvated Zn ion complexes and create a supersaturated inner layer electrolyte to drive uniform metal deposition

• Organic additives (e.g. SDS) change surface morphology and surface orientation
• LiCl–ZnCl2 (water-in-salt) mixture-concentrated electrolyte on the hydrogen bonding interruption of water molecules and found that Zn2+ ions can coordinate with Cl− rather than H2O, leading to strong O–H covalent bonds while decreasing the solvation activity of H2O in the electrolyte

Bromine half-cell

In a Zn–Br flow battery, the fundamental positive-electrode electron-transfer reaction is usually written as :

• 2$\text{Br}⁻$ → ${\text{Br}}_2$ + 2e⁻ (Charging)
• ${\text{Br}}_2$ + 2e⁻ → 2$\text{Br}⁻$ (Discharging)

However, $B{r}_3^{-}/B{r}^{-}$ appears in the literature because once ${\mathrm{Br}}_2$ is formed in a bromide-rich aqueous electrolyte, it rapidly undergoes the equilibrium ${\mathrm{Br}}_2+{\mathrm{Br}}^{-}\rightleftharpoons {\mathrm{Br}}_3^{-}$

So the same cathode chemistry is often written in the overall form:

• $3{\mathrm{Br}}^{-}\to {\mathrm{Br}}_3^{-}+2e^{-}$ (Charging)
• ${\mathrm{Br}}_3^{-}+2e^{-}\to 3{\mathrm{Br}}^{-}$ (Discharging)

Conceptually, at the electrode bromide is oxidised to bromine. Then, in the electrolyte, bromine is redistributed into dissolved/complexed bromine species, especially ${\mathrm{Br}}_3^{-}$, and at higher bromine loading also higher polybromides such as ${\mathrm{Br}}_5^{-}$ and ${\mathrm{Br}}_7^{-}$.

In practical batteries much of the bromine inventory is stored in these complexed forms rather than as “free” ${\mathrm{Br}}_2$.

Electrolyte

On top of ${\text{ZnBr}}_2$, supporting secondary salts (e.g. ${\text{ZnCl}}_2$ and KCl) are normally used to promote ionic conductivity and lower internal resistance due to the low conductivity of zinc–bromide solution, thereby increasing the battery’s energy efficiency

Membrane

• Must minimise the diffusion of ${\text{Br}}_2$ to the electroplated zinc, which causes self-discharge and lower CE for the system
• In terms of morphology, membranes can be generally classified into porous and ion-exchange membranes (e.g. Nafion®), which are both appropriate and capable of separating the anode and cathode electrolytes in ZBFBs.
  • Porous membranes are defined as macroporous (> 50 nm), mesoporous (2-50 nm) or microporous (0.2-2 nm) depending on their average pore diameter, while non-porous membranes transport ions via solution–diffusion mechanisms

Characterisation (page 22)

• Operando measurement = taken while a battery is operating (cycling)
• In situ measurement (meaning on site) = measuring a variable against a parameter relevant to the system, such as time, temperature, pressure or other variables, to minimise its degradation