Have you ever wondered why batteries hold less charge the longer they are in use? Have you ever wondered how they can be made to last longer? For traditional lithium-ion batteries, the answers at the heart of these questions are partially found in understanding energy material degradation that occurs while the battery is in use, or, in other words, while the battery is charging and discharging. As you charge and discharge a lithium-ion battery, changes in a multitude of properties, like increasing electrolyte viscosity and side reactions, can heighten electrical resistance and lower the capability of the battery to store charge. In order to study the effects of these physical and chemical changes, researchers turn to experimental thermodynamic tools, such as calorimetry or potentiometric entropy measurements. These tools allow researchers to find the sources of heat losses and determine how the battery components change as the battery cycles—that is, as it charges and discharges. Heat losses and chemical changes indicate battery inefficiencies, and information about these inefficiencies can lead to better-informed battery design.
Calorimetry and entropic measurements play a role in battery electrode design
As a battery discharges, the chemical process of lithium flowing from a system of high chemical potential to low chemical potential allows for a conversion of chemical potential energy to electrical energy that can be used to power a car, a phone, or other electrical devices. During discharge, lithium ions at the anode are oxidized, freeing an electron to do electrical work. Lithium ions flow through the electrolyte and a semi-permeable membrane to the cathode. When charging the battery, an external voltage is applied to allow lithium ions to travel back to the anode. Figure 1 shows a schematic of the charging process.
In commercially available batteries, this transfer of electrons occurs concurrently with a multitude of uncontrolled processes. As batteries are cycled multiple times, sources of degradation and electrical resistivity begin to emerge. These sources include increasing viscosity of the electrolyte, undesirable side reactions, and phase transitions at the electrodes.
In determining which lithium-ion chemistries can be good candidates for high charge rate or high-capacity battery materials, it is useful to understand what happens as the battery charges and discharges, or in other words, as lithium ions enter and leave each electrode. Researchers in the Synthetic Control Across Length-scales for Advancing Rechargeables (SCALAR) EFRC have been using calorimetry and partial molar entropy measurements to look at the voltage profile and extent of these changes1. Calorimetry and entropy measurements—both thermodynamic tools that rely on changes in heat and temperature respectively—can provide a lot of information on how fast a material degrades and changes as well as how much heat is generated with lithium insertion and de-insertion.
Calorimetry gives information on sources of heat generation, which can be used to predict electrode material lifetimes and determine the cause of many battery inefficiencies2. There are multiple sources of heat generation, but three large ones come from processes called Joule heating, heat of mixing, and side reactions. Joule heating occurs when an electrical current flows through a conductive material, like metal wires. The more resistive a material is, the more heat generated from Joule heating. Therefore, knowing how much heat is produced from Joule heating informs researchers about certain points during charging or discharging where the battery electrodes are too resistive. The heat of mixing is heat generated by a resistance to mass transfer. It tells researchers whether there are large gradients in the lithium concentration throughout each cell component, which can further inform researchers whether the material can benefit from different processing techniques. The heat generated by side reactions sheds light on unexpected reactions occurring between different cell components. For example, the heat generated by side reactions shows whether either of the electrodes are reacting with the electrolyte solution3.
The partial molar entropy can be used to determine how lithium sits in the electrode1. As lithium ions go into an electrode, while the battery charges or discharges, the lithium ions have preferences for where they want to sit in the material with some degree of periodicity. This periodic pattern and the structure that surrounds are an example of a phase, a physically distinctive form of matter. When the battery charges and discharges, the phases of the material change. As the phases change, either the partial molar entropy or voltage profile change. Therefore, when combined with other techniques, changes in the partial molar entropy inform researchers of the battery electrodes' changes.
Examining a high-performance anode chemistry with entropy and calorimetry measurements
TiNb2O7, also referred to as titanium niobium oxide, is an anode material found to excel at fast-charging applications, reversibly cycling at a rate of 10C, a rate that corresponds to a six-minute charge time, while keeping a capacity of 150mAh/g. Graphite, the material that makes up the active material of most lithium-ion battery anodes, quickly degrades at 10C. The high operating voltage for TiNb2O7 limits the production of a solid–electrolyte interphase layer at the electrode surface that is caused by degradation of the electrolyte and overall leads to slower Li+ ion movement at the solid–electrolyte interphase. Additionally, this material is safer than conventional graphite anodes. This is because operating at these higher voltages can help prevent lithium dendrite formation. Lithium dendrites are small, sharp growths of lithium that can puncture the membrane that separates the cathode and the anode. Puncturing the membrane results in the battery short-circuiting and can lead to runaway reactions that can cause fires.
Researchers at the SCALAR EFRC applied calorimetric and entropic measurements to study the heat generation and lithium-ion ordering in this anode material1. Since it is a recently studied material, first cycled in 2011, there is little information on what occurs as a cell containing this material charges and discharges. What researchers found was that, while cycling at high rates, Joule heating dominated energy losses. The high proportion of heat generation occurring through Joule heating indicates that the main source of inefficiency was from electrical energy losses and that further research should focus on ways to reduce the material’s resistivity. The heat of mixing stayed slow at even high charge and discharge rates, which corroborates why this material is a good anode for fast-charging materials.
Outlook for thermodynamic techniques in battery design
Calorimetric and entropic measurements are effective tools for dissecting the sources of energy losses in a battery. These techniques are both useful in determining which battery chemistries are suitable for specific applications, as demonstrated with the fast-charging anode material, TiNb2O7. The measurements also provide insight into which inefficiencies are present in individual battery components so that engineers can target these energy losses in a material-specific fashion.
