Table 2.

Summary of performance metrics, cell attributes and assessment methods of ZBRBs

	Description	Significance and definition
Performance metrics and cell attributes
Cycling stability	The ability of a battery to maintain its capacity and performance over multiple charge and discharge cycles	Both cycling life and stability indicate the battery degradation mechanism
Cycle life	The number of charge and discharge cycles that the battery can withstand before its capacity degrades below a certain threshold
Coulombic efficiency (CE)	A measure of how much of the energy charged into a battery is available for use during discharge	Provides a useful measurement of charge reversibility CE = Qdischarge/Qcharge × 100 [144]
Voltaic efficiency (VE)a	A measure of how much energy is available from a battery compared to what was charged into the battery	VE = ${\overline{E}}_{\text{disch}}/{\overline{E}}_{\text{ch}}$ × 100 [153]
Energy efficiency (EE)	The amount of energy the battery can deliver relative to the amount of energy it consumes during charging and discharging	EE = Ev,cell discharge/Ev,cell charge × 100 [153]
Power densityb	The rate at which the battery can deliver energy	P = (I × Vcell)/A [20]
Energy density	The amount of energy that the battery can store relative to its weight	Ev,cell = $\frac{\int \text{d}{Q}_{\text{discharge}}\times \text{d}{V}_{\text{cell}}}{V_{+}+V_{-}}$ [154]
Theoretical electrolyte capacityc	An electrolyte's theoretical capacity stored in a given volume	Qt = n × c × v × F/3600 = ncv × 26.8 (mAh) [20]
SoC	The charged capacity stored over the calculated theoretical capacity	SoC = Qcharge/Qt [155]
	Output and applicability	Challenges
Assessment methods
Electrochemical liquid phase transmission electron microscopy (EC-LPTEM)	Understanding the mechanism of the dendritic growth In situ	Commercial batteries may experience significant deviations from normal operating conditions when using special probing systems with modified battery structures
Transmission X-ray microscopy (TXM)	Detecting dendrite formation in minimal architecture zinc–bromine batteries In situ	Requires special cell structure to be used for in situ measurements
Potentiometric titration	Detecting the concentrations of the redox-active species in ZBRB electrolyte solutions Ex situ	Time-consuming
UV/vis spectroscopy	Measuring the electrolyte species concentrations Determining the SoC of the RFB electrolyte In situ and ex situ	Difficult to obtain accurate SoC estimation value in ZBRBs as the active materials exist in a two-phase hybrid system
Raman spectroscopic analysis	Detecting the total concentration and the SoC of the zinc–bromide electrolyte for the real-time SoC estimation of the negative electrolyte in ZBRBs In situ	Not quantitatively suitable for the positive electrolyte with the oily polybromide complex phase that is dispersed non-homogeneously
Fourier transform infrared (FTIR) spectroscopy	Determining the electrochemical double layer at the SEI on carbon-based materials (e.g. glassy carbon) Determining the storage complex reactions and rates of some organic BCAs (e.g. MEP and MEM) In situ	Requires more studies on different carbonous electrodes (e.g. graphite and carbon felt) with different properties (e.g. surface area)
X-ray photoelectron spectroscopy (XPS)	Providing detailed information on the chemical composition and electronic structure of the surface Ex situ	Requires special cell structure to be used for in situ measurements

^a ${\overline{E}}_{\text{disch}}$ average voltage over discharge, ${\overline{E}}_{\text{ch}}$ average voltage over charge

^bI is current (mA) and V_cell is cell voltage at a given current (V)

^cn is the number of electrons per mole, c is concentration (mol L⁻¹), v is volume (mL) and F is the Faraday constant