Batteries are energy-storage devices and segmented at a high-level into the primary and secondary battery types. Primary batteries provide a single discharge. Secondary batteries are capable of being recharged.
Li-ion batteries are today’s leading technology in this space, driven by their high operating voltage, up to 5 Volts. Other technologies, Li-Air, provide higher theoretical capacity operation. CYCLIC VOLTAMMETRY is used in the development of the material and identification of its potential window (charge cutoff voltage and discharge cutoff voltage). CYCLIC VOLTAMMETRY is also the primary technique in identification of new electrolytes. The Princeton Applied Research PARSTAT 3000A is particularly suited for this phase of research as its form-factor is specifically engineered to be capable of installation into a glovebox. The BNC connections of both the Solartron EnergyLab and PAR potentiostats (by the optional cell cable) provide the interface common for glovebox feedthroughs.
Localized techniques, available on the VersaSCAN platform, are at the fore-front of research to characterize the Li-intercalation mechanisms and Solid-Electrolyte Interface (SEI) formation.
As the goal of a secondary battery is to provide a high cycle-life, high efficiency, and high energy density; new materials are combined into a complete CELL and evaluation is a CHARGE-DISCHARGE experiment. A variation on this technique is Constant Current – Constant Voltage (CC-CV) where the battery is held at its charge cutoff potential and trickle charged to ensure a fully charged state.
The goal of these experiments is to determine the CAPACITY vs CYCLE NUMBER to evaluate cycle-life and COULOMBIC EFFICIENCY – to quantify charge in / charge out (Qin / Qout). CAPACITY represents a measurement of charge in the engineering units of Ah. This translates easily to define the time that a given device can deliver a given current.
For example, a coin cell (CR2032) size battery is commonly 200 mAh. This device then can be discharged at 200 mA and expected to last for 1-hour. This time is normalized to a battery’s capacity and reported as a rate (in terms of C, 1 C = 1h^-1). This is the most common size for evaluation at this stage, as it is easily assembled even in academic laboratories. The next step up is the 18650 size battery which is commonly 2250 mAh (2.2 Ah). This device then can be discharged at 2.2A and expected to last for 1-hour. The Battery Holders for the current line of PARSTATs and VersaSTATS allow for direct connection of both of these battery formats to the potentiostat. Direct connection avoids the stray, added capacitance and inductance to impedance measurements. Replacing the cell cable creates a cleaner signal and a cleaner lab. The EnergyLab EIS methods, including its innovative FRA-technology and oversampling, allows for characterization of devices of sub-milliOhm (microOhms) of impedance.
As single cycles of these experiments will last hours and the goal is to determine capacity fade in the >1000 cycles range; the value of a multichannel platform and early predictive screening cannot be overstated.
The flat-voltage profile, seen as a key advantage of Li-technology, drives the need for advanced techniques to determine STATE-OF-CHARGE. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) is the emerging method for making these determinations in-situ. EIS is also used to determine how the battery is functioning with respect to its anticipated lifetime, referred to as STATE OF HEALTH. See our published guide and technical explanation here: Link. The full range of products from Princeton Applied Research and Solartron Analytical provide these measurement capabilities either as standard or as options. EIS also provides a mechanism via Equivalent Circuit analysis or simple visual reference to identify the Equivalent Series Resistance (ESR) of a battery. This is a key figure of merit as it represents a loss of the system.
The use of auxiliary voltage measurements allows monitoring of both the Anode and Cathode of the battery. Standard potentiostat design concentrates on the signal and response at the Working Electrode. The Counter Electrode reactions are not characterized; while other applications use an inert Counter Electrode; in battery technology, this is an active electrode. Being able to characterize this terminal additionally allows users to identify failure mechanisms and properly focus research initiatives. This is available on the PARSTAT 3000A and EnergyLab products for single cell evaluation and PARSTAT MC (PMC-2000A) and 1470E CellTest for multiple, simultaneous tests for improved throughput.
For extreme applications that require greater than 5 Volts or 2-Amps of current, batteries can be configured in stacks. A stack is the term used when several singular devices are connected in series or parallel for higher power applications. Since stacks are purposefully designed for operation at high voltages (up to 100 V) or currents (up to 100A), external boosters are required. External boosters are available is a wide range of measurement capacities, bandwidth and accuracy to meet a given testing profile.
The value of the auxiliary electrometer is further evidenced by the need to identify when a battery within the stack starts to fail. The PMC-2000 and PARSTAT3000A provide the voltage range to test a stack of batteries as well as the standard, additional electrometer to measure the characteristics (including impedance) of a single battery within the stack. The 1470E and EnergyLab provide multiple electrometers to study even more cells within the stack.