A charge-discharge cycle involves draining or using your battery to where there is for all intensive purposes, no charge left, and then subsequently charging the battery with a power adapter to 100% capacity. This process of charging and discharging (charge cycling) can only be done between 300-500 times. The question that we want to address is why? Why is it that lithium batteries can only be charged less than 500 times? Why does a battery over time degrade and eventually stops working and what if any does the reduction of the battery’s active material and subsequent causes of chemical changes effect battery degredation?
Battery Degradation and Power Loss
A battery over time degrades and eventually stops working, this is no surprise, but why this occurs is really a fascinating yet technical process. These reasons are complex issues that are way beyond user control and are wholly contained within your battery and within your device! These technical processes are a result of the reduction of the battery’s active material and subsequent causes of chemical changes. The chemical changes that I write of are:
Declining capacity – when the amount of charge a battery can hold gradually decreases due to usage, aging, and with some chemistry, lack of maintenance.
The loss of charge acceptance of the Li‑ion/polymer batteries is due to cell oxidation. Cell oxidation is when the cells of the battery lose their electrons. This is a normal process of the battery discharge process. In fact every time you use your battery a loss of charge acceptance occurs (the charge loss allows your battery to power your device by delivering electrical current to your device). Capacity loss is permanent. Li‑ion/polymer batteries cannot be restored with cycling or any other external means. The capacity loss is permanent because the metals used in the cells run for a specific time only and are being consumed during their service life.
Internal resistance, known as impedance, determines the performance and runtime of a battery. It is a measure of opposition to a sinusoidal electric current. A high internal resistance curtails the flow of energy from the battery to a device. The aging of the battery cells contributes, primarily, to the increase in resistance, not usage. The internal resistance of the Li‑ion batteries cannot be improved with cycling (recharging). Cell oxidation, which causes high resistance, is non-reversible and is the ultimate cause of battery failure (energy may still be present in the battery, but it can no longer be delivered due to poor conductivity).
All batteries have an inherent elevated self-discharge. The self-discharge on nickel-based batteries is 10 to 15 percent of its capacity in the first 24 hours after charge, followed by 10 to 15 percent every month thereafter. Li‑ion battery’s self-discharges about five percent in the first 24 hours and one to two percent thereafter in the following months of use. At higher temperatures, the self-discharge on all battery chemistry increases. The self-discharge of a battery increases with age and usage. Once a battery exhibits high self-discharge, little can be done to reverse the effect.
Premature Voltage Cut-Off – some devices like PDAs do not fully utilize the low-end voltage spectrum of a battery. The pda device itself, for example cuts off before the designated end-of-discharge voltage is reached and battery power remains unused. For example, a pda that is powered with a single-cell Li‑ion battery and is designed to cut-off at 3.7V may actually cut-off at 3.3V. Obviously the full potential of the battery and the device is lost (not utilized).
All batteries degrade and lose power because there is a reduction in the battery’s active material.
We know that a battery is a device that converts chemical energy into electrical energy. In order to convert chemical energy into electrical energy there is a chain of events that have to occur prior to the creation of electrical energy. The chain of events have been discussed in depth in previous articles which you can access on my blog but what is key to the creation of electricity is that in batteries electrical energy is produced from two chemicals in a solution. After discharging you recharge the battery via a charger. The charge process involves intercalation: the joining of a molecule (or molecule group) between two other molecules (or groups). Intercalation is the process of ions being pushed by electrical current into solid lithium compounds. Lithium is one of the chemical components used to create electrical energy in batteries. Lithium compounds have minuscule spaces between the crystallized planes for small ions to insert themselves from a force of current. Ionizing lithium loads the crystal planes to the point where they are forced into a current flow. Intercalation replenishes, in effect, lithium but the net result of ionization is the ultimate depletion of the lithium reactive property. You could say if you use it you will lose it!
Why then is lithium used as the chemical to create electricity in batteries? There are a number of good reasons – let’s look at a few!
General Characteristics of Lithium
- Name: lithium
- Symbol: Li
- Atomic number: 3
- Atomic weight: [6.941 (2)] g m r
- CAS Registry ID: 7439-93-2
- Group number: 1
- Group name: Alkali metal
- Period number: 2
- Block: s-block
- Standard state: solid at 298 K
- Color: silvery white/grey
- Classification: Metallic
Lithium is one of the metals in the alkali group (the other metals include Sodium, Potassium, Rubidium, Cesium, and Francium). Lithium is a highly reactive metal. Lithium has only one electron in its outer shell (two electrons in its inner shell), which makes it chemically “ready” to lose that one electron in ionic bonding with other elements. Lithium is used as a battery anode material (due to its high electrochemical potential). Electrochemical potential is the sum of the chemical potential and the electrical potential. The higher the electrochemical potential the better the electrical current yields. In some lithium-based cells the electrochemical potential can be five times greater than an equivalent-sized lead-acid cell and three times greater than alkaline batteries. One other core advantage that lithium has is that it is soft and bendable which allows for tight configurations in small cell designs (PDAs. Laptops, Cameras etc…).
Lithium, even with all of its good chemical properties will eventually, however, react to the point where the electrochemical potential will yield a charge that is simply not enough to create current to pass to power a device.