Charging of Lithium-ion battery
Lithium Ion batteries are the most widely used battery types for portable electronics. The major downside such as damage of cells due to rapid charging that Lithium ion batteries carried during their initial days of development are a thing of past. Present day Lithium ion batteries are highly robust and have rapid charging features associated with them. The basic science behind Lithium ion batteries is that lithium ions move from negative electrode to positive electrode during discharge or powering up other devices.
Types of Lithium ion batteries:
Based on the chemical composition, Lithium ion batteries can be of multiple types such as Lithium iron Phosphate (LiFePO4), Lithium ion Manganese Oxide battery (LiMn2O4, Li2MnO3, or LMO), Lithium cobalt oxide (LiCoO2) and Lithium nickel manganese cobalt oxide (LiNiMnCoO2 or NMC). Except for Lithium cobalt oxide Lithium ion batteries, rest of the battery types offer low energy density. However, they all have a longer life and more safer when compared to Lithium cobalt oxide batteries. Lithium cobalt oxide batteries although have high energy density, they pose safety risks which is even truer in the case of damaged ones. Lot of research has been carried out on Lithium ion batteries in order to make them safe and also to increase their life.The older version of Lithium ion batteries were more fragile and were prone to damage during charging due to the fact that they get overheated while getting charged. Thanks to the extensive research on these batteries, techniques have been discovered to ensure that batteries get charged at a rapid rate without getting abnormally hot.
Inside a Lithium Ion battery:
The electrolyte, the positive electrode and negative electrode constitute the lithium battery. The positive electrode can be made of a lithium cobalt oxide or a polyanion such as lithium iron phosphate or a lithium manganese oxide which is a spinel. The negative electrode is usually made of graphite. Lastly, the electrolyte is usually a mixture of organic carbonates such as diethyl carbonate containing complexes of lithium ions or ethylene carbonate. During the discharge of battery, movement of ions happens through the electrolyte from the negative electrode to the positive electrode. This causes electrons to move in the opposite direction to power. A reverse movement of ions happens while the battery gets charged. Intercalation is insertion of molecules in into compounds that have layered structures. Present day cells use lithium based intercalation compounds such as lithium cobalt oxide because of the fact that it is much more stable than pure lithium. Ions during their movement, react with the electrode and get depleted quickly during the course of electrochemical reaction. This reduces the life cycle of battery. Also, each time the battery gets charged, the electrodes get expanded in voluminous proportions which severely affect the crystalline structure causing microscopic damage and gradually reducing the ability of electrodes to lodge free ions.This has a huge negative effect on the number of recharge cycles. Addressing the above issues have been the primary objectives of research into lithium ion batteries. The solution that was made available to address this issue is to pack more lithium ions into the electrodes in order to increase the energy density which facilitates free movement of ions in and out of electrodes thus easing the passage of ions through electrolyte.
Chemistry within Lithium ion cells:
The combination of Lithium and ion have some basic advantages when compared to other combinations. Lithium has the lowest reduction potential when compared to other elements. This fact allows Lithium based batteries to have the highest cell potential. Also, Lithium is the third lightest element having one of the smallest ionic radii of any single charged ion. All the above translates to the fact that Lithium batteries have high gravimetric capacity. Gravimetric capacity is the measure of total charge capacity stored by the cell per gram of battery’s weight. It is measured in mAh/g.
Deep dive into the anatomy of Lithium ion cells:
The outermost layer of the lithium ion cell is made of metal casing with a vent hole that is pressure sensitive. The purpose of the pressure vent is to release any extra pressure in order to prevent the battery from exploding. The positive electrode and negative electrode within the battery are separated by a separator. The separate while separating both the electrodes, allows ions to pass through. Both the case sheets are submerged within an organic solvent that acts as an electrolyte.
It is no brainer that the higher the capacity of the battery, the more time it requires to get charged. A 2000 mAhbattery gets charged completely within less time when compared to a 4000 mAh battery when supplied with a current of 500 mA. It is true that increase or decrease in charging current is inversely proportional to increase or decrease in the charging time. However, this is true till a certain degree beyond which increased current will get dissipated in the form of heat which rises the internal temperature of the battery causing severe physical damage. Increased charging may also lead to good number of ions getting embedded into the negative electrode to such an extent that the electrode disintegrates to the point of ruining the battery.
Science behind rapid charging:
We need to understand two terms before getting into the details of rapid charging.
- Constant Current: Constant current is time independent DC or Direct Current that does not change its intensity with time
- Constant Voltage: Same voltage input provided to the battery through the course of charging process
The science behind the present day lithium ion batteries ensures that the battery gets charged at a rapid rate without accommodating the negative effects of rapid charging such as overheating or physical damage. Till the time the battery reaches 4.1 or 4.2 V, it is charged at a constant current of 0.5 C or less than that. After reaching 4.1 or 4.2 V, the charger automatically switches to the constant voltage phase where it is supplied with same voltage during the entire course of charging in order to eliminate overcharging. The constant current of 0.5 C or less drops gradually till it reaches 0.1 C. Charging is terminated after reaching this point. So, one may ask what happens if the charger is not turned off. If the charger is not turned off, a periodic ‘top up’ is applied to counteract battery self-discharge. This top-up occurs when battery reaches a point that is less than 3.9 to 4 V. This top-up charge gets terminated once the battery reaches 4.1 or 4.2 V capacity. Overcharging and undercharging can prove costly to the overall life of battery. As ions have finite mobility, overcharging may not proportionately help increase the movement of ions, causing conversion of electrical energy applied into thermal energy. This thermal energy can cause physical harm to the battery and sometimes may also lead to explosion due to outgassing of electrolyte. On the other hand undercharging the battery by as little as 1% can reduce battery capacity by 8%. Refer to the image below understand the reduction in battery capacity due to battery undercharging.
Hence, manufacturers of batteries advise users against overcharging and undercharging batteries. Some of the modern batteries also include temperature detection mechanism in order to stop charge charging after reaching certain temperature to prevent damage.
Best practices with Lithium Ion battery:
We need to ensure that the batteries never reach a deep discharge state of zero as it ruins the life of battery. The batteries need to be charged after its partial discharge.
Lithium Ion batteries should never get overheated as overheating ignites the electrolyte resulting in catching fire. Also, if the separator that is present in the battery gets ruptured and if the electrodes touch, the battery heats up real quick.