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Is Li ion battery safe?

The answer is yes if quality cells are selected and used with proper cautions. We have occasionally seen about Li ion cell failure in fields. It has sometimes injured people and raised safety concerns of Li ion cells. In order to use the Li ion cell properly engineers need to understand what the critical points are for the safe design of battery packs and devices.  .

Safety factors related to Li ion cell chemistry

OVP (Over Voltage Protection)

The electrolyte of Li ion cell is EC (ethylene carbonate) based organic solvents. The organic solvents start to be decomposed (electrochemical oxidation) when the voltage is over 4.4V. Over 4.5V the decomposition is accelerated rapidly. The decomposition of electrolyte generates ethylene, propylene and carbon dioxide gases. The gases build a pressure inside cell and result in venting of cell.

The standard charging condition of Li ion cell is 0.5C rate CC (Constant Current charging)/4.2V CV (Constant Voltage charging). When charging voltage and charging current are increased over 4.3V and over 1C rate respectively, the transfer of Li ion from electrolyte to graphite is difficult. It increases a chance to plate Li metal on graphite surface. The plating of Lithium is called as dendrite (needle shape). The dendrites may penetrate separator when impact, crush or high rate discharging is applied. When the two factors (electrolyte decomposition and internal short due to dendrite formation) described above are combined, the cell will fail in catastrophic way that is explosion with fire.

Most of Li ion cell in the market have anode limiting design. It means that the capacity of cathode is higher than that of anode. When a cell is charged with trickle mode which is used for NiMH battery charging, the cell voltage will continue to rise. If trickle charging is continued, anode cannot take anymore Li ion from cathode. The excess of Li ion forms dendrites on anode surface. It also can generate internal short of cell and explosion. Therefore trickle charging should not be applied to Li ion cells.

There are 4.25V and 4.35V OVP values from protection IC makers. Under 4.2V, 0.5C CC/CV charging conditions, 4.25V OVP is recommended. Under 4.3V, 1C CC/CV charging conditions, 4.35V is recommended. However please keep in mind that 4.3V charging with 1C charging current may lower the cycle life of cell.

UVP (Under Voltage Protection)

When Li ion cell is over-discharged less than 1.5V, graphite (anode) structure is destroyed. Graphite has a tunnel structure. A minimum of Li is required to hold the structure. Over-discharged cell is charged, the cell voltage is not increased. It means that graphite becomes like conductive plate. As Li ion cannot go to tunnels inside graphites, Li ion plates on graphite surface and generate Li dendrites. When dendrites are formed, there are always safety issues. In order to prevent safety failure of Li ion cell due to over-discharge, UVP, pre-charging and battery charging disable voltage should be applied. 2.5-2.7V UVP values each cell are recommended. Between 1.5V to UVP of cell voltage, pre-charging should be applied. 1.5V charging disable voltage is recommended. However these values may be changed according cell manufacturers. Each cell manufacturers may have own formulation or chemistry of cathode and anode. It gives somewhat different values.

Battery parameters that show manufacturing reliability

Most of design engineers have not known much how cell manufacturing process affects the safety of Li ion cell. Selecting a good supplier is the first step for safe devices. Deviation of cell capacity, impedance and OCV (after aging) clearly indicated manufacturing processes.

Capacity

Capacity of cell is simply determined by the weights of anode and cathode materials. The processes that affect the capacity of cell are mixing, coating, and calendaring. The key is how to manufacture uniform thickness electrode. If the electrode thicknesses are not uniform, the capacity of cell from one to another is varied. Capacity variation (18650 cell) of the first tier companies showed within 50 mAh in a lot.

If cells with very different capacities are used to build a pack, the pack will give a short cycle life and may give some safety problems potentially.

Impedance

Impedance variation from one to the others cells indicates uniformity and reliability of manufacturing process. The variation comes from winding, electrolyte injection, welding process and external contaminations. Impedance variation of the first tier companies is within 5-10 mohm in a lot. Manual winding and electrolyte injection may give more than 20 mohm deviations. If impedance data show several very low values compared to the average value in a lot, it may indicate a soft short due to pin hole of separator by external metallic particles or sharp edge of anode copper foil. This can create a hard short that may results in cell explosion. Further if impedance data show several very high values compared to the average value in a lot, it may indicate low electrolyte contents or welding defects between Jelly roll and cell container.  In order to build more than 4 cell packs, impedance variation among cells in a pack should be controlled within 10 mohm.

OCV (Open Circuit Voltages)

When cells are aged, self discharge rates of cell are varied depending on cell chemistry and internal cell defects. OCVs of cells decrease as a function of time. OCV changes as function of time is the very useful way to detect external metallic particles. During slitting process, some copper particles can be generated. Any copper metallic power can dissolve in electrolytes and forms dendrites when cells are aged. As most of cell manufacturers have used separator of 17-30 micro meter thickness, very fine particles can break the separators. This may result in cell explosion.  When some cells show very low OCVs compared to the average OCV value, it may be internal soft short. Furthermore the deviation of OCV changes indicates the uniformity of cell manufacturing process. The OCV deviation with the first tier companies is within 0.01V. When a pack with more than 6 cells is built, OCV variation among cells in a pack should be controlled within 0.01V.

Li ion cells can be used safely if reliable cells are selected and proper protections are applied to cells and packs. A number of cases due to safety failure have been reported since Li ion chemistry was introduced in 1991. Many of accidents can be avoided if design engineers understand safety parameters more.